Exciting Project! Can't wait to see it in action!

I think this is pretty cool, have you thought about how you'd adapt the EMG signals to work with different people - the excitation's can be pretty different between individuals.

The Myoware sensors actually have an in built gain control so you can take into account the different signals people produce. The device will always have to be calibrated for people but it's a very easy process and doesn't seem like a problem.

This is so cool! just an idea, you could add an option to make one sensor perform a double stroke roll.

I'm not sure what you mean by this? A double stroke roll requires 2 "hands" in a LLRR or RRLL pattern

Do you mean to double to tempo you could program it to do two strokes per movement? This would be interesting but might be difficult as the delay between the two hits would have to perfectly match (or be double) the tempo that the user is contracting their muscles at!

I think two sensors would be better as Matthew commented below.

How quickly can it work? If it's not possible to get a rapid enough rhythm it might not work for most songs - maybe having two sets of sensors and motors would allow for more rapid drumming?

This is a really good point, the fastest i could reliably get it to go with single beats is 80BPM, most music is above this so in theory two sensors could achieve 160 which is well above most popular music.

I hadn't thought about that, you could alternate which sensor reads an input at any given time. 

Would I need a second arduino to achieve this? If using the delay command then during the delay no other reading can be taken. This would mean that the sensors could not function simultaneously. Maybe a clock function could be used to avoid delay or some custom functions could be written and called.