I'm not sure the religious/mythic question would necessarily define their interaction. When Heortling Uroxi and Praxian Storm Bulls meet, they are likely to see each other as foreigners first, and may never realize that they might be co-religionists. They probably don't speak the same language, and given the nature of both cult manifestations they probably won't try to talk to each other. Also, given that the Heortlings don't follow the way of Waha and the Praxians don't honor the laws of Orlanth, both sides would see the others as "outside the law" even if they do share some cultic similarities.
My picture of life around the Block has never included any sense of deep unity, even between Praxian Storm Bulls. I've always seen them as forming tenuous alliances to fight chaos, which frequently break up due to tribal differences. My guess would be that foreign Uroxi would be even less likely to interact in any way besides heavily armed neutrality.
On the other hand, kvass and mead make for strange bedfellows...
Gerald