CGA347 Nipple with a F-F-F Tee on each bottle... Short 1/4 NPT M-M between them. Suitable whip from the compressor to the first tee, fill whip out of the other tee would let you fill from either bank (using the tank valves), or direct from the compressor, or fill from the bank with the compressor running, or fill both banks from the compressor.