There wouldn't be any other charges, unless there's an hour limit (I think usually dialup has it as about 360)

I don't *think* you can usually connect two computers to a single account even with two phone lines, and if you could it'd probably violate some TOU.

But it's been a long time since I tried to do that, and my memory's fuzzy. I think it got tangled up in the phone lines I had going all across the house

aside of that, you could always learn to share and or change sleeping hours and work hours to get internet time.