I have 2 phosban reactors on my 33 cube, the first one is for carbon, and the second for phosban. I daisy chained them together, and using the flow rate that is best for the phosban, which is on the lower side for carbon. But, IMO, that is better then having ideal flow for carbon which would be too fast for the phosban.
I know that this isn't ideal but works well for me, and I only have to use one pump. I but the carbon first because I change that more often and with the way mine is set up it is easier to get to that one.