The way that the cable splitter by problem solvers (which is NOT cheap, by the way) works would make 'adding' force by grabbing both brakes less effective than you might think. Yes, it would work, but the question you should ask is, to what end? There is a point when clamping the rim that too much force will simply lock it up; very uncomfortable on a bicycle with only a front brake. A properly setup front brake of decent quality has no issues stopping/slowing a bike with a decent squeeze. Properly setting the brake up (good quality lever/caliper/pads and paying careful mind to use good quality housing and good installation practice) will go very far in making a brake work acceptably.

There is likely at least some miscommunication with this statement, but in my mind, no brakes, regardless of quality/quantity on a bicycle will stop it quickly (compared to, say, on flat ground) going down hill. Physics is working against you in that regard. It takes quite a bit of energy to go from 30mph down to 0, quickly.

Perhaps you can post a picture of the front brake you have?