When you have two pensises, you don't ask why, you just shut up about it and enjoy having two pensises.

It's actually a hemipenis. It's a single penis with two heads. Forked like their tongue. It's because they breed in piles and it doubles their odds of hitting a target

Probably to make mating easier, since they can protrude out on either side, rather than the snake having to reposition, or end up poking straight down into the dirt.

But genital shapes can be pretty weird in general. Just look at humans. Instead of a simple tube, they have a squishy plug thing on one end. They don't even have a baculum to keep it straight, they just use hydraulic pressure.