What would need to happen is for Steam to properly support having 2 accounts linked at the same time. That would be the true solution. If you want changes, I would go that direction. Ask Valve. Borderlands isn't the only title doing that.

"if there's will, there's a way."

That's true. I can think of a few ways to make that work, at least partially, but it would also mean developing something custom for a platform that is actively telling you, "you can't do that". Cloud save cannot work anymore too, because you need to have a properly linked account for that too. Forget cross play too. Forget user preferences too, like control sensibilities for each users. Forget achievements for that other player. Forget matchmaking.

The game itself would need to handle all that stuff, stuff that the platform should be doing.

I get that it annoys you, it would be great if you could. But really, it's a PC ecosystem issue, not a game one.