We use relay.damus.io, nos.lol, and our own relay (relay.trotters.cc). Three relays gives enough redundancy that signing requests get through even if one drops.

For NIP-46 specifically the relay does not see anything useful. The messages are encrypted between client and bunker, so you are just looking for reliability and uptime rather than trust. The two you are using are solid. We added a third because we had a couple of timeout issues when a relay was slow to respond and the signing request just hung.      

If you want to test, the easiest check is whether your bunker's signing round-trip stays under a second or two. If it does, your relays are fine. If you are getting timeouts, adding a third relay as fallback is cheaper than switching.