The inevitable happened, albeit sooner than we had hoped. The downstairs baby gate is no longer a deterrent of any sort to Bandit. We've also discovered that Bandit is quite the athletic jumper! He wants to be with us at all times (as is typical of the Aussie breed), so he discovered how to take a big leap without any running start.
We figured this was as good a time as any to give him free run of the house. We can't tell if Simba is excited or a little apprehensive over this new arrangement.
EDIT 7/24: Bandit might be ready for being upstairs, but we're not ready for him to be up there. K went to Petco today and got a new gate which is much higher than the old one. This one has a little door for us to go through. And in time, when Bandit is bigger, there's a tiny little door for Simba to utilize. Right now, Bandit is such a size that he'd be able to make it through the little cat door, no problem.
We just discovered that Simba is fully capable of jumping "over" this new gate, so this little kitty door isn't totally necessary for him, although he might find it easier to use in the future