I have that in mine because thats just what it is on paper (two 1189 tunze nano's and a 775gph return pump on a 30g long). I've also had a single 2100gph maxi-jet mod in there and feel pretty comfortable saying it caused more havoc as it was virtually impossible to position where I could use it without blowing something around too much. I lived with a single 1600gph prop in there long term without problems. two small prop maxi-jets are going to be sufficient for a 75g sps dominated mixed tank trying to keep lps and other corals and I'm confident two 2100gph props will be enough regardless of what your doing. Four maxi-jet mods are typical for a 120g (4'x2'x2') which is significantly deeper and wider than a 75g.
While I would agree if and I stress IF the pumps were on full time. But the OP wants to use them on a wavemaker which will only have 1 at a time on and thus the flow is not truely 4200gph or 8400gph but infact is 2100gph + the return at any given moment.