Rehid stores its key mappings in .json files located either as sd:/rehid/rehid.json for global mappings or in a title ID folder within sd:/rehid/ to use with a specific title. You can manually create your own key mappings by creating a rehid.json file with the key mappings you want.
Documentation
"keys": Maps one or more keys to press different key(s).
With the above, any time you press the ZR key, R key would be triggered, and any time you press the ZL key, L key would be triggered.
The keys you can use for this are: A, B, X, Y, SELECT, START, ZL, ZR, L, R, LEFT, RIGHT, UP, DOWN, CLEFT(CPAD), CRIGHT(CPAD), CUP(CPAD), CDOWN(C-PAD), CSLEFT(C-STICK), CSRIGHT(C-STICK), CSUP(C-STICK), CSDOWN(C-STICK)
You can use this to do custom key combos, such as the following: