Interesting topic that I have pondered,without great results,for years. Like many,I come from the B3 era,and also wanted to transition to keyboards. My first rig was a hollowed out Gulbranson spinet with 2 Peavey KB3 cabs. I was 23 in "64,so the weight was relatively light. In the years since,I tried 2 platform stands,but the reach killed me,even though the hight differential was fine. The second tier stands,as we know,don't cut it,but close on some.
Today,I mostly have forgotten the idea,except with my Korg MicroKey Air. It is so thin and narrow,one can place it in front,at the same level. There is always a but--,and that is fooling with midi. I can midi it into the main keyboard,or,bring my SD40.
No real solutions,just a bunch of past memories.
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pa4X 76 ,SX900, Audya 76,Yamaha S970 , vArranger, Hammond SK1, Ketron SD40, Centerpoint Space Station, Bose compact