Generally speaking, the gods do pay attention to who's who and to people entering their domains! Anybody who has previously attracted heavy attention would be noticed and kicked out quickly.

However, this does work on a scale. The gods are more-or-less omnipresent in Ryslig - so if someone is very open about information (such as being a god follower) in Ryslig, both gods will know! However, they don't have omniscience, or the ability to pay attention to everything all the time. If someone does not seem to have been very dangerous to them, or has not ever worked against them directly, a god may pay less attention to their entry.

It is certainly possible - if very difficult - for a careful opposing follower to sneak around, as long as they do no damage.