I have a colleague who is looking for a phrase, idiom, quote, saying, etc. that basically conveys the idea that “a smart person will make smart choices individually, but two (or more) smart people make stupid choices when acting together.” I’m sure there is one, but I just can’t think of it myself. Any help?
A person is smart. People are dumb, panicky dangerous animals and you know it.
- Agent K, Men in Black
A camel is a horse that was designed by a committee.
Too many cooks in the kitchen?
Yeah, God still regrets letting the Holy Spirit sit it on that design session 
Pithy saws usually reflect things that are true, at least quite a lot of the time, but what you are looking for seems to me to be, generally speaking, not true at all. I should have thought that it is actually much rarer for smart people to make stupid decisions when working together with others than when when working alone.
Here are a couple. The second isn’t very pithy but it certainly does express the idea. Just delete the last line 
I think the one closest to what you’re looking for is going to be the one from Sanity Challenged however.
A committee is a thing which takes a week to do what one good man can do in an hour. ~Elbert Hubbard
Could Hamlet have been written by a committee, or the Mona Lisa painted by a club?.. Creative ideas do not spring from groups. They spring from individuals. The divine spark leaps from the finger of God to the finger of Adam. ~Alfred Whitney Griswold
Yes but the OP does not ask about creative works, which are the complex products of many subtly interacting choices, but about individual decisions.
And, I would submit, a large part of the problem with committees is that, more often than not, they do not consist entirely of smart people. Also, although committees can surely have their failures, there are actually good reasons why they are so widely used for making important decisions. Generally speaking, (and even when all the members are not geniuses) they do a more reliable job than individuals.
Right… but “smart” and “creative” are pretty different things.
I’m with njtt: as presented, the OP’s colleague’s idea strikes me as strange and wrong. I’d like to hear an example of its truth.
The one I have heard is - People are clever. Crowds are stupid.
“Two or more smart people working together” is not a “crowd,” either.
Groupidity?
One quote I found was: “A committee is the only form of life with many heads and no brain.”
I prefer my own formulation: “Intelligence isn’t additive, but stupidity is.”
AFAIK, this is original with me
Not sure what your goal is so this might be irrelevant, but research shows that 2 to 3 heads outperform 1 significantly.
Yeah, I don’t expect any phrases like this to be true all of the time or even most of the time, but perhaps cynically, such phrases can serve as useful tactical tools when necessary.
To determine the IQ of a group, take the lowest individual IQ, then divide by the number of people in the group.
(Larry Niven, maybe?)
Not a phrase, but a related phenomenom to what the OP is looking for is groupthink. The classic example of groupthink is the Bay of Pigs fiasco, in which a group of very smart American planners thought they could overthrow Castro’s government with a group of ill-prepared Cuban exiles.
Why does the last post of a thread always have the idea that I came in to say? It’s called groupthink.
This author argues just the opposite to the OP’s question: The Wisdom of Crowds: Why the Many Are Smarter Than the Few and How Collective Wisdom Shapes Business, Economies, Societies and Nations
Slightly tangental to the OP but one of my favs has always been that “Nine men cannot produce a baby in one month; that’s because men don’t have babies”.