In the studio I have a triple rack with the Yamaha PSR9000 on the bottom. It has a hard drive, a CDR and 64 Mgs of Sample Ram. The middle rack holds a KN6000, (never could get a hard drive to work in it). The top rack has an Ensoniq Sampler, EPS 16 plus Turbo. There are other keyboards and modules that are midied through a MX8.
The only problem is a music rack can only go on the top keyboard or a free standing to the side.
I use a single rack with a keyboard and the music holder when I need to get real serious
Bebop