I know Sam Andrews and survival sheath both make dual-gun shoulder rigs.
You're not going to get effective fire while riding, and (maybe more importantly) you're not going to be able to ride effectively while firing. And if your antagonist is driving as well, his fire won't be very effective, nor will his driving, so you should take advantage of this to get the hell away. (Esp. if your antagonists are also on motorcycles, like your 1%ers.) Your gun isn't going to mean a thing, nor will you retain it, if you're holding it in one hand and drop the bike on top of you or go over the handlebars.
Just my two cents, and I've been brainwashed by the US government, but they train us drivers to keep driving until the vehicle can't go anymore. Then it's time to think about using the gun, if you're still alive and conscious.