ssjos,
I forget specifics, but it's set in such a way that if you are both making 3000 beakers, and you merge, together you will both make just more than 3000 beakers. However if you are making 3000 beakers and he is making 1000, together you will produce just over 2000.
Basically, take your average beakers, and add a small amount (I think less than 10%) to get your total beakers to be applied to the next technology.
This is also a good strategy to use against the AI if you ever play team games. Destroy someone and leave them with only one city. Chances are they'll only hold back their partner's (or partners') technological development.