What would happen if you grew 2 plants in one big pot?

You'd have two plants in one big pot, simple as that!

They wont necessarily kill each other because firstly it dependes on width depth and capacity of the pot. If the pot is wide enough the two plants can be spaced apart leaving enough room for their roots. If the pot is deep enough and hold enough compost,their roots will go down deep,but if the pot is shallow and do not hold enough compost their roots will grow outwards and tangle with each other and stunt their growths.

as long as the bin or pot or whatever they are in is big enough to suport both of thier root systems they will be just fine ( asuming your buddy waters them enough and such). only once they get big enough will your bud need to think about spliting them up or mving them to a bigger home. and they wont just sudenly die. if they need to be split up your buddy see it coming asumeing they know at least a little about keeping plants alive.

dunno about coffee but about mile from my home there are growing 2 different trees which uses each other to survive supporting each other. in valentine day those who where in love even had a fest near those two. (looks like they're in love, lol)(if someone wants i can take a shot of that later ^^ )

They will compete with each other for nutrients and water, and likely be weaker than if they were separate (unless larger amounts of water and fertilizer were provided). But they are unlikely to kill each other through competition.

the roots would grow together and you couldn't move them apart again. if the pot was big enough they would be ok