Severe + other crit upgrades

When I was playing Novitiates the other day and had a minor disagreement with my opponent about how Severe interacts with the 2 point Act of Faith that also upgrades a hit to a crit. My opponent argued that I couldn't use the Act of Faith after upgrading a hit to a crit with Severe to get two crits. I couldn't really find a clear argument either way, but I thought that if Severe works with Punishing and Rending then there should be timing where retained dice can be changed after Severe applies.

So... can you "change one of your normal successes to a critical success" with Severe, and also "retain one of your normal successes as a critical success" with a different rule? Does it only work with some wordings? Are there rulings or other guidance from GW about this?