No , you can't work simultaneously and if you want to go for IAF go for it only. There is no requirement to be a IAS then IAF. Both of the service are a Grade A Gazetted post so you should bother about any of the stuff. You should go for what you are passionate for. If you like to be in Public Administration then prepare for UPSC if not then don't waste your time in it. Go for IAF.

I hope my answer helps you. All the very best for your future endeavors!