It doesnt really matter very much, as its often just to far to reach out comfortably... the keys keys are no problem, but the controll surface is just to far to use continously...
I never liked two keyboards especially playing out ......I don't like pedals either I might only use a double Boss FS 5U linked together for Sustain/VH thats it, just two awkward to reach up and play a top kb, for me,....I'm an
all in one kb player, using my fingers is faster then the foot Imo,.....at home is a different story but who wants to carry two kbs around on the road? YUK!!..its a personal choice for each player whatever makes you comfortable is what you should use,....but it's certainly NOT for all......I remember years ago on stage stacking 2 kbs on top of my B3, Rhodes/Juno 60,..

but times have changed and so have I,

....good luck in whatever rig you create music with....
there are no rules....take care..