The guidance cantrip says the following (PHB, pg. 248):

You touch one willing creature. Once before the spell ends, the target can roll a d4 and add the number rolled to one ability check of its choice. It can roll the die before or after making the ability check. The spell then ends.

The Combining Magical Effects section says the following (PHB, pg. 205):

The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect--such as the highest bonus--from those castings applies while their durations overlap, or the most recent effect applies if the castings are equally potent and their durations overlap.

If there are 3 PCs (Alice the Cleric, Bob the Druid and Charlie the Rogue), and two of them know guidance, can they both (Alice and Bob) cast it on another PC (Charlie, who is about to make an ability check) such that Charlie can use the better of the two d4 rolls, effectively rolling the d4 "with advantage"?

Obviously the d4s don't combine; rather, if the d4 from Alice's guidance rolled a 1 and the d4 from Bob's guidance rolled a 4, then Charlie gets to add the 4 instead of the 1 to his ability check?

No, they don't

In the same section (PHB, pg. 205) that you've quoted, it also gives the following example:

For example, if two clerics cast bless on the same target, that character gains the spell's benefit only once; he or she doesn't get to roll two bonus dice.

(Emphasis mine)

This effect is similar to the Guidance spell, so Charlie will only get one extra d4.

The wording most potent effect in this scenario does not refer to a roll result, but rather to the number of dice, or a fixed effect. For example, if you cast a spell that gives a 2d4 bonus, it will overrule a spell that grants a 1d4 bonus.

Fore more information on overlapping spells and which one to choose, check out this question

Working together is better than trying to stack guidance

Having the PC attempting the task add 1d4 to a roll with advantage beats trying to stack 1d4, which you can't do anyway due to the combining magical effects rule that you already cited.

I have done this in a number of situations where my cleric casts guidance on our Barbarian, and then another of the players provides help which gives the Barbarian advantage on the Survival roll (foraging for food in a wilderness adventure) or on the Athletics roll (in a case where we wanted to make sure that we knocked down a door).

The average bonus from guidance is 2.5 (rolling a 1d4), while the expected benefit from having advantage is somewhere between +3 to +5, depending on the task's difficulty. (See this question and answer for an exhaustive treatment of what that benefit of advantage is when rolling 2d20.) What you could expect as a benefit is guidance's +1d4 and a further + 4 (+/- 1) depending on the DC of the task. (While I particularly like RSConley's answer, all of the answers are useful)

Working Together

The character who’s leading the effort—or the one with the highest ability modifier—can make an ability check with advantage, reflecting the help provided by the other characters. (Basic Rules, p. 62, italics mine)

Have Bob help Charlie after Alice cast's guidance. Charlie 1d4, and rolls with advantage.

Caveat: unless both characters have proficiency with thieves tools, this won't work on picking locks.

We have also combined advantage and guidance to avoid getting lost in Chult's jungles/forests (Tomb of Annihilation published adventure) where before I make my Wisdom(Survival ~ Navigation) check, the cleric cast's guidance. I have advantage already due to my favored terrain being forest. (That's how the DM rules it. I can see how another DM would require a different approach (Ranger helps the hired guide, cleric casts guidance on the guide, unless the ranger PC is from Cult originally.))

No, you can't add the bonuses together.

According to Sage Advice, game features (class features, spells, etc) with the same name don't stack with each other. So no. Casting bless twice at the same PC won't let them have 2 rolls.

...But partially Yes: you get to choose the higher value roll.

Reading the Combining Magical Effects section as RAW, yes you choose the highest value if you cast the same spell. Even if you don't subscribe to the rules, logically, if the DM allows, if 2 PCs' literally role-played casting bless on a 3rd PC at the same time, you can waive of the lower value of the 2 rolls as an error in following the rules. And even if the 2 PCs unintentionally cast bless at the same time frequently, without malice, you should still check on the DM if they'll allow waiving off the lower value.

Almost - you can do something similar

Using RAW something similar should be possible. Since there are two overlapping effects, only one is active at a time. Once this spell ends, the effect of the second one will become active again. But since the spell ends, once you use the bonus you can use the second spell after that.

So you can do the following RAW:

1. Frodo wants to lockpick a door


2. Two people cast guidance on Frodo


3. Only one of the two spell-effects is active, the other one is suppressed


4. The player of Frodo rolls a d4 -> rolls a 1


5. The player decides to use this bonus on a different ability check, e.g. jumping as high as he can.


6. Now one of the two guidance spells ends after the effect took place.


7. After that the suppressed second spell becomes active, since there is no more overlapping effects


8. Now Frodo can roll his d4 for the second guidance spell -> rolls a 3


9. Now Frodo chooses to use that 3 for his lock picking.

No... but:

In situations where multiple ability checks need to be rolled it could come in handy.

If you cast double guidance on Jerry the sneaky rogue (or Steve the not-so-sneaky paladin) for example who might need to make multiple Stealth checks within one minute; and in that case having double guidance would be fairly helpful.

And I also can see situations where the fighter is grappling someone and the druid and cleric are just standing over him casting guidance every round because they can't do anything else. That would be fun!