Are you talking about flying after you get out of CRNA school? Or flying while you're in CRNA school?
Obviously, you'd still qualify as an RN...I don't know what your background in nursing is, so I don't know if it's a good flight background or not. You won't make any more money flying than you do nursing, so I don't know why you'd want to fly instead of pass gas to pay off CRNA school. You can make a lot more money as a CRNA. If you just want to fly first, then I'd recommend getting a flight spot for awhile, then pursuing CRNA after you've spent some time flying.
One of the flight nurses where I work is a CRNA. He was a flight nurse before he went to CRNA school and he stayed on as a casual after CRNA school. He only works a few shifts a month as a flight nurse.
Would be a total waste to go to CRNA school just to be a flight nurse, besides CRNA school wouldn't really be a preperation for flight nursing. I can't imagine a nurse manager hiring a CRNA to be a full time flight nurse. They would have to be thinking that the CRNA is going to get tired of making penuts and leave sooner or later.
Consider the educational preparation. As a CRNA, your expertise will be the administration of anesthesia and management of the peri-operative and immediate post-operative patient. This does not necessarily translate to the knowledge needed for managing patients during flight. Now, on the other hand, doing flight nursing first could help you prepare for a CRNA career down the road. The autonomy of flight nursing combined with the advanced skill set and critical decision making skills would be a big plus for anyone looking to apply to a CRNA program.