Many posts here seem to imply that having such a double brake setup is going to give you more braking power than a normal brake. This is not the case.

Assuming that your V brake and disc brake are set up to engage at exactly the same point, and they both need exactly the same pull ratio to brake (either of which is unlikely), then the braking power you get is somewhere inbetween the braking power you'd get from either of them. Obviously the force you put into the lever will be distributed across both brakes, and not multiplied.

If the brakes are not set up to engage at exactly the same point, or the pull ratio is different, effectively one of the brakes will do most of the work and the other will be completely or almost completely useless. Or you might get a non-linear brake response (e.g. if the weaker V brake engages first, and the stronger disc brake later, or something like that), which would also suck.

Having one lever for both the front and the back brake might be marginally more useful, but it's not a good idea either because having individual control over the brakes is important when riding on slippery surfaces.

I suppose that this brake lever is designed for trikes, but I could be wrong because I know nothing about trikes.