We thought about the baby-gate too, but we did have the concern of them
jumping over. I guess we could do a trial run with TWO baby-gates...I'm a
sorta new uncle...so they're sorta new grandparents who just got done
babysitting for a weekend.

We could probably set them close to eachother and up a few steps. Maybe we
could set them close, but one step apart? That way we'd be able to remove
it going up or down. Has anyone tried this?

And I'll keep in mind the fresh air. They are indoor cats, one will just
sit by an open door, the other runs away from the door. The one that runs
away was abandoned by it's mom outside my wife's (then girlfriend) house.
It meowed so much that she took him in and nursed him back to health. So I
think he has issues with going outside.

What about using a baby-gate at the bottom of the stairs? If using a
baby-gate, I would probably put it a step or two up from the bottom step,
since it seems it would be harder for a cat to jump over the gate that
way.
Or you might consider a bi-fold closet door--the type that accordian when
you open them, but you wouldn't be able to move that one.

If you don't already have plans to, you might also want to consider
getting
harnesses and leashes for your cats so you can take them outside
sometimes.
Just carry them upstairs so they don't get used to climbing up and down
themselves (so they don't think it's "OK" to do so). Cats need fresh air
sometimes, too :-).

rona