This is not an exception, the 1-foreigner-per-heya rule is for new rikishi, not those ending up in a heya, with a foreigner already, after a merger.
That's how some heya can have several foreign-born rikishi, like Kataonami.
If Isegahama merged with another stable with a foreign rikishi, like Oshima-beya, it would end up with 3 foreign-born rikishis.