The answer is actually quite simple, the perfect companion for a Kronos would be the new tyros 5... Because it adds what the kronos lacks. Great arranger styles out of the box, and awesome acoustic instruments (sa voices) which is in my opinion the weakest point of the Kronos.
If you want 2 keybaords tyros and kronos is the way to go, espescially if you allready own a tyros 4/5. Personally i would go for a 76 T5 and a 61 kronos. however the 88 key Kronos has one of the best piano keybeds available, so espescially if you are a piano player, this could be a great option.
[video:youtube]
http://youtu.be/Wq-Czb-Ramg[/video]
Hee is a video explaining the advantages of Kronos and casio PX-5S from the viewpoint of a pro musician.