Whether there is room or not on two retainers depends upon what else you have on them. I've got one retainer that is borderline full (bounces between 160 and 175 slots filled depending upon my materials usage and Venture materials refill rate) with nothing but SB materials, and another that holds the few odds and ends needed in SB crafting from HW/ARR. I foresee that retainer getting the bulk of the stuff for the starred recipes when I move into those, but that second retainer has maybe 30 of the 175 slots filled? Plenty of room for expansion.

The trouble comes when you try to overstock your retainer (do you really need that third, fourth, fifth, sixth, and seventh stack of Molybdenum Ore?) or when you try to load the retainer up with other stuff in addition to the crafting materials.