If you separate RW from FW, what do you do next? Separate FJ from Heavy from Light? Civil trained from military? For that matter what do you do with the handful of talented individuals who were trained in one and have migrated to another, or exist in small niches of only two or three specialist Test Pilots?
That said, a non-FT related association that I belong to, the IMechE has a simple approach to this - annually we all are asked to fill out an "activities" questionnaire with our membership form. That creates membership of "groups" within the society - entirely by individual choice - and determines which of the IMechE's many sets of meeting notices, technical newsletters, etc. land on the mat. That presumably wouldn't be at-all difficult for SETP to do?
On the other hand, we were told at the LA symposium last year that SETP needs to reduce the proportion of SCUM in it's membership. That is Southern Californian US Military ! If the society succeeds in that (by, I hasten to add, bringing more of the others in, not throwing anybody out), then most of the concerns others have mentioned above seem to be met in some way.
There is incidentally a helicopters group within the RAeS for those looking for a serious home for helicopter boffinry, but it's membership is all aerospace professionals, not just TPs.
G