I have (a) above and it does its job OK, but whilst you can adjust the angle of the second keyboard you can't adjust either its height above the main one nor how far forward/back it is. Personally I'd rather have it flatter, lower and further back, which isn't possible with this attachment.
It does level off for the "flatter" adjustment but I guess you may be concerned about it sliding off or moving if it is not slightly inclined forward so it rests in the grip of the front hooks.
How well it fits your preferences also depends on the keyboards.
The PX350 is not deep but it is tall
in height so it brings the top and bottom keyboards closer yet I think I would like the 2nd tier about 2"-3" above the bottom (just enough to get my hand between them. I do not need access to any buttons other than the On/Off switch.
Once you decrease the space between the top and bottom keyboards you need the ability to move the bottom forward and the top back.
Because the Casio is not very deep, the depth of the base stand has room for me to slide the Casio forward. The MOX is also not as deep as the top tier so there is room to slide it back though it would no longer rest on the front hooks. That could be compensated with two pieces of black pipe insulation from Home Depot.
The lowest setting of the base stand is higher than ideal with the Casio being taller. The height was perfect when the MOX was on the lower level.
As it is now I can use it and the footprint is much more compact than the larger 2-tier Z frame I tried.