Why are you testing old existing work with water? Theres a LOT of places you could have a leak and never see it under the slab.
Yes, you will need to plug the other closet flange too.
Make sure to get your test balloons down past the lead or you could expand the lead under the slab.. You don't want to make any added holes to that system.
I am replacing the cast iron plumbing traps in the two bathrooms. I don't think there's enough room to add a test tee between the new p-trap and the existing cast iron I am tying into. Here's a shot of the new pvc tie in.
In order to water test the new traps, I think I will have to plug the downstream two way clean out in the yard. The two toilets tie into the mainline, which is why I am trying to figure out how to plug. I am worried the existing lines will not be leak free, but uncertain what else to do.
Is there an inflatable plug that I could get downstream of a 2" PVC trap? Then I wouldn't have to worry about all the potential leaks in the existing lines.