As long as youre not talking about primary O2s on a single turbo, I have been running my secondaries on a single sensor for over a year now. Twins of course.
There is a toggle for it i believe in tuner pro. It uses bank 1 sensor 2 but you still have to leave bank 2 sensor 2 plugged in from what I've read.
I run this way because I've been too cheap to buy another for bank 2 sensor 2. Rather get a wideband setup instead.