I had mine set up to piggyback a 7.5hp motor along with the 15hp.  I built a bracket to mount the 7.5 above the 15, then put a 4 belt pully on the blower and a 2 belt on the 7.5.

 That setup worked very well, if I remember it gave me another 10-20% more capacity.  I started the 15 first then pluged in the 7.5, I had it all wired together so if something happened it would all shut down.  When i started the 7.5 you could see the amps fall!

 I still have all the pulleys, shield(home made), motor mount(home made), and belts.  I do not know where you're located but I would like to get rid of them if intrested, since I went up to a larger drier and 5" system.