So you have 2 x 1inch drain pipes coming down from the tank? According to the calculater on RC, a 1 inch drain pipe comes out to about 600gph max so your drains could theoretically handle 1200gph max. Right out of the box your Pan World 150 is running 1100gph and the Iwaki another 1500gph. There will be some headloss due to all the plumbing, but you best do some calculation work to make sure you aren't pushing more water into the tank when both pumps are on than what the drains can handle.