That's when you just spam hearts and calorie producers. Also, what would happen if you built two brains for the same system, or two brains connected when their little patches of ground connected? Would they fuse together, or simply communicate?
I think, if they were made by the same player, then they'd be more likely to join together; working separately on their own things, but collaborating where they see fit. Though, the closer they are, the more likely they are to simply (in terms of mechanics, not physically) fuse together, an average of the fused brains psychology scores taken, and used as the new values, with a significant boost to their overall intelligence. The more independent and sentient a brain is, the less likely it'll be to fuse with another brain, but the more sentient and intelligent (and perhaps friendly) it is, the more likely it it'll be to work together with a new brain its encountered.
Then, you could end up, on a SMP server, with a huge community of independent, but interconnected, brains working together towards a common goal. I, personally, think that'd be more than a little kickass. Well, that, or it'd lead to one colossal, multi-core brain taking over the entire world, and perhaps even reaching into alternate dimensions.