I'm not sure why you want to run two instances on one computer, but I can answer your question:

For some reason (and I have no idea why!), PROCESSING the downloaded blocks from the swarm is slow.  Validating the blocks shouldn't be too slow, but if you have some way of checking, you'll see that the node that is downloading blocks is slamming the disk.

Without actually profiling the software, I can't say why, but I believe you should just be able to copy the block DB (blk*.dat) from one instance to the other without any badness happening.  Just be careful not to blow away your wallet!