Right On Pro!
I used to split 50/50 with a girl singer. She was definately a crowd pleaser and had a following. But, I did feel a bit put out by the fact all she had to do was show up (although...she did spend money and go to extra trouble with her apparel, makeup, etc.). I did all the arrangements, bookings, owned and maintained all the gear, upgraded gear, did afternoon setups and stayed to tear down while she was long gone, etc.

Unfortunately, she became ill and could no longer sing. I thought I was sunk. But, when it all washed out, I didn't lose but a couple of jobs when I went single. In addition I hired another girl as a "standby" for those times I needed to provide a duo. The new girl only works with me on about 10% of my jobs and is happy to get the work at far less than the previous singer. I always try to be fair...but no more 50/50.

Al...in all honesty, your biggest challenge is that you have established a precedent. And precedents can be very hard to change and cause ill will. If you were building a new band from scratch, I don't think you would find any objections from your band mates.
Eddie