Sitting on another players lap and they can't get off the bike until you get off the bike, and you're the one driving; he's probably being ironic, but even if he is it sounds terrible.
Scenario: You stop your bike in berezino for a moment, just to take account of what your plans for your run are, and some freshy runs up to you and jumps on your lap and now you've been kidnapped. Is that how you want to spend your time in the game?
I'm not saying two people can't ride a bike together, but they generally don't try to share one seat, if you've never tried to ride a one seat dirtbike with two people, it's dangerous and you wouldn't try to drive fast and do stunts. Maybe if they had pegs to stand on it would be safer.
Someone else mentioned that the backpacks could clip and that would be unrealistic, in a game that favors realism. I understand that backpacks already do this in cars. We shouldn't embrace such mechanics.
That being said, it might be fun to do in a game what you can't in real life, and trying to ride a dirt bike with two people would be fun, and risky. I'm not saying that we shouldn't have those things, my point is that these bikes don't appear to be designed to seat two people at the same time. (And it may be more of a challenge for the devs for other reasons that we don't know, which relate to complicated mechanics in the game engine.)
Once this update is pushed to stable, keep an eye on the workshop, I would be willing to bet that someone is going to make the bikes fit two players and make it more dangerous / risky.
You could easily make @nate_lapt's feature proposal a reality, just allow a second person to get on the bike, removing the ability for the first driver to control the bike when this happens. The two player models would overlap, and remove the ability for the first driver to get off the bike when this happens. Hypothetically, this is a very simple mod.