350pounds put you right up to the edge of (possibly over) what a ninja 250 is designed to carry. Legally speaking, you are not allowed to operate a vehicle above it’s max load weight. And there are pretty good reasons for that. If you plan on doubling up, I’d look at a Ninja 500 instead.
Don’t be afriad of getting a used 500 (or a suzuki GS500), they haven’t had any significant change in close to a decade. If you can save yourself a few hundred bucks by getting one with a few thousand miles on it.
Thanks, and keep safe over there.