I want to distribute two versions of my app, the stable branch as well as the current development trunk, using TestFlight. And, if possible, I want to invite the testers only once.
Can I have two versions of one app in one TestFlight team? Or maybe two app with different namens? Or can I create a second team and link it to the first one or copy the testers over?
Unfortunately I think there is no nice way to do that. Your options are:
Two different TestFlight teams. You'll have to invite people to both teams. But, TestFlight is clever and if it already knows about a user in another team who is in the provisioning profile in the IPA you upload, then you select that they can access the build, it will auto-invite the user.
Use different app IDs for your stable and development branches.
I would personally go for the first option.