On question 1, I suspect you can drive it indefinitely with a bad O2 sensor. There may be longer term issues with your emissions, and if you live in a state with required emissions checks, a “check engine” light on may be problematic for registration.
Another concern is, if you leave the O2 sensor as is and the check engine light on indefinitely, you might not know when/if another problem arises. In other words, your check engine light can only come on once, whether you have 1 problem or 10.
As for question 2, I’m not sure. I tend to think that if one sensor has gone bad, the other one might not be far behind. Replacing both at the same time, to my mind, would depend on the cost of the second sensor.
It’s none of my business, but I’d go ahead and get the problem seen to sooner rather than later. But I tend to like to knock down “problems” as they arise rather than letting them snowball.
Good luck.