the 2 water outlets is really gonna cut flow.. on mine.. my head is around 3 1/2 feet which puts it at around 300 gph.. with a U around the top which probably cuts it down to 225-250
So if you have a similar setup with a 7.. you might only be putting out 200gph out each outlet.
Is your tubing 5/8" or 3/4" inner dia?
I dont think you can pump more than what your tank can drain thus may be just have one return spout. Even though similar amt is being pump, but may see stronger circulation/output from one return than two returns.
take some pics of your plumbing if possible...is the tank drilled or you have an overflow?
im guessing you have a hang on overflow...which is probrably the one that can handle 600gph(has a 1" bulkhead for drain)
then the 700gph mag7 should be enough....unless you plumbed it with alot of 90 degree elbows...
i would like to see how your sump is baffled...so we can determine the type of flow needed in the sump....remember you dont want too high of flow going through your sump so your protein skimmer can extract the proteins more efficently...(if you have a fuge in-line then you want medium flow through there so the macro algae can time to absord some nutrtients)
with a mag7 pushing out into 2 return nozzles...you will not feel like its that strong....but remember ...the water is diverted into 2 areas..not like its pushing out of only 1 nozzle...like another guy posted earlier....use powerheads to add flow in the tank...pick up 2 koralias and that should solve your problems...they use very little watts and gives great wide spread circulation...if you plan to use the return pump for circulation/flow in the tank then you'll need something in the 1000gph range which will drive up the electric bill...(return pumps are mainly used to return the water from the sump)...ok...we need more info from you and you need to do some more research....enjoy