Plumb two Mag's in series (one feeding into another), either 7 or 9.5. Same speed, usually 1.5 - 1.8x more pressure. (and don't plumb ANY aquarium pump in parallel ever, they're not designed for that)
In case you don't believe me:
http://www.engineeringtoolbox.com/pu...ial-d_636.html
On the other hand, if you want good quality pumps, go for Reeflo Pumps. Reeflo also recommends doing the same thing if you need more pressure. A bit fancy for your use, but reliable. Either way, make sure your plumbing fitting are solid, as higher pressure means even small holes may start dripping.