a) Having two dies and thus improving YIELD on the manufacturing process is better than going for a monolithic design with 30% yield. ATI probably learned from this with their R600.

b) CrossFire interconnect seems to be handled internally. The drivers take care of all that - it's not like Crysis or other games that handle CF bad will say "Oh no 3870 X2 time to stop working" Windows sees the card as a single card, not a crossfired set of GPU's.