if this does actually happen, with both of them actually giving 100% effort (this is for Bolt more than Rudy) , it will probably be closer than you all think.
Bolts open 400 PR is 45.28 , given it was back in 2007 but his training was also more 2/400 focused around that time
Rudishas open 400 PR is 45.5 , also awhile back and probably an extension of his training regimen more than anything.
as for Bolts mythical "43.6 split", keep in mind this is a split in a relay, and although he did indeed run sub 45 (~44.5 ish) there is no way Bolt is in the same 400 shape as he was in 2010, check out his first race this year. Rudisha can run sub 45 at his best, never mind the fact that hes skinny, the man is an animal.
there needs to be a sizable cash incentive to win and give effort, and we could see a truly great showdown