It's a very good question! It made me curious to measure the width of my two hands (along the knuckle line), which turns out to be 7.5" (I have large-ish hands). And this exactly matches the length of my fully aroused cock - what an interesting co-incidence!
A few times I have used both when jacking off, but only when I'm about to cum and taking those final deep thrusts - and I also hold my hands against something solid (bathroom sink counter-top, for example) so I can thrust with my body. But, most usually, I use only might right (favored) hand to keep the left free (and also lube-free) to either hold the remote control, squeeze my nipples or balls, or play with my buddy if we're jacking together.