The best way ( I think ) to use two arrangers at the same time is to NOT midi them together. Let each one be in control of it's own destiny, and set them up to give you more variation than one can provide.
I only want to have ONE set of controls, rhythm choices, etc.... but I wouldn't mind having two playing surfaces every now and again. Most (99%) of my work only requires one kb, and two would be a deterrent, but there are those rare instances that can really be "punched up" with a dedicated piano or organ. ( my taste )
If I had to narrow it down -
Top two choices for high end arranger would be 9kpro and kn6500.
Top two dedicated synth/piano would be -
RD700 and hmmmm....... can't think of a better 88 key choice at the moment.
(although I LOVE my FP3 for lots of reasons that I've stated before)