I have been running 3 m770's inboats off one JL m6450 ( approx 150wrms each) for two seasons. I got this idea from David at Earmark. 6 inboats total on two m6450's. They work well and I havent encountered any issues.
An mhd 750 on only two m770's is a lot of power for each of those, but used responsibly they should be fine.
My one complaint about using the mhd750 on two tower speakers is that they are operating on the same channel. Earmark David and I have had discussions about this topic, I feel that some of the audio effects are lost when only using one channel, David disagrees. I use my stereo mostly for partying on the boat (close range listening) I like the sound separation of two channels on the tower vs mono.