It is possible that I have never heard this line before.
It is a fantastic line full of possibility (Are these debates actually leading to head headbashing, brainspattering fights? Or does all that intellectual discussion just create an atmosphere of heady carnage? Are the brains being thrown about literal or figurative or both?) but its place in this conversation about the boys’ companies means that it is cut 9 times out of 10. If not 10 out of 10. If the little eyases stay in at all, usually, it’s their first mention and then on to Hamlet’s line about his uncle.
And with good reason. A discussion about a rivalry happening a long way away from the action of THIS play is completely inessential to the story. And given that this play runs about 4 hours at a steady clip uncut – most people find it beneficial to lose this sort of conversation first. But I do love the image of throwing about of brains.
I get the sense that Guildenstern has a somewhat dry sense of humor. It makes me like him more. Especially since he has this line that conjures up a zombie food fight.
The Hamlet Project 
My current morning coffee reading project, in the hopefully latter stages of the Covid Years, is reading Hamlet. That reading project began two years ago with a new appreciation of poetry and other literary styles that I tended to avoid in the past.
Nevertheless, while my initial approach to Hamlet tends towards “what does he **mean**”, it was the sheer poetry of this phrase that got me here through google. Your entry validates that instinct (alas, I still need validation from others for my ideas.)
One of my other Covid Times activities is rather obsessively participating in heated forums and boards where, indeed, there is much throwing about of brains. Can’t wait to use the quote.