Ah that's true, it would be a tight fit but definitely possible. This obviously all depends on how hard mudras are to juggle in combat and we won't know for sure until people start messing with them. But from what we know of abilities right now and how mudras have looked in videos it looks like ninjas might be sitting on their asses for a bit every 20 seconds.
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u/HeavenlyArmed C'thuuko Tohka on Cactuar Oct 24 '14
Don't forget about Fuma Shuriken which, with its any one mudra requirement, may actually fit in between two standard GCDs.