based on some writing I've found in internet, it said that the 6th cog function in our stacking is indeed dominant. So, based on that writing (if valid and everybody could verify), it is possible to use Ti along with Te, Fi+Fe, and so on.
Say you're ENFP, then the stacking :
Ne Fi Te Si // Ni Fe Ti Se, so it is possible to use Fi (as aux) and Fe as "another dominant"