TPG,
If it is a 1/3 hp you should have no problem adding a second diffuser. Depending on the CFM requirement of the 9" you have alredy installed, a 1/3 hp piston compresso should produce 4.4 cfm at 30 psi. if it is a Thomas Wobl compressor.
The 9" you already have most likely uses only a maximum of 1 cfm so you have extra cfm to use for a second diffuser.
By the way welcome.
