There's a couple nearby, the guy rides the bike and the lady sits on a rack, side saddle.

They're both very petite, so that works in their favor. I'm almost sure they used this rack.

So long as you've a decent rear-wheel and strong axle, you'd be fine.

Bending/breaking an axle isn't a big deal, they're very inexpensive and readily available. Less than a round trip for two on the bus.

A couple of places here in the states have a law against second passengers on bicycles not built/equipped for that task (child seat/Xtra cycle style.) If that's not the case in your area, I'd go for it.