I think the answer lies here http://www.mattcutts.com/blog/myth-busting-virtual-hosts-vs-dedicated-ip-addresses/ but I don't get what Craig is trying to say?
Well if you are using only two domains then it would certainly not matter much, even it matters a little! Matt qouted in his blog: Take care Manish
He is saying that "Links to virtually hosted domains are treated the same as links to domains on dedicated IP addresses." In other words, same IP or multiple IP's, there is NO difference.
If anyone is still curious, heres a video by Matt Cutts that address the question http://video.google.com/videoplay?docid=3583760678227172395