It might be, but you can't cross at every level.

I'd allow a bit more time than the normally conservative dhuey has suggested. I'd also suggest parking in the Int'l G garage as it is much easier to maneuver, it saves a few $, and the walk from your car (even with elevators) to T2 security is maybe 10 minutes. From UA back to the G garage is even quicker on your return.

If time is precious, the off-site garages might welcome you with a "do you have a reservation? If not, go away" issue, at which point, you'll end up on-site anyway.