Two arrangers...., in one set up..??? I know everyone's needs are different, but you could easily use the S-900 as your main board, and buy another performance keyboard to add extra sounds. There are several keyboards out there without sequencers ect.., but are nice (add ons) to an existing set up.

Again.., everyone's needs are different, but I'd suggest looking at an affordable performace synth.... They have great sounds, and are an inexpensive way to add "new and different" sounds to your existing arranger. Plus they're lightweight too. Just a suggestion.