I know of the potion cooldown glyph bug, but suspect that the glyph that extends potion effects may not work either.
Created a 15.7 second invisibility potion, stayed away from anything to suppress it.
donned two rings with extension glyphs, 3.5 and 3.6.
Drank potion from quick items and immediately checked status, which was 12 remaining.
Even if rings don't stack, I did not take more than a few seconds to get to the status screen.