use two 87/88 not mythical 97s and almost double your numbers of fills use tanks like theses two add this doing it this way still allows you to unplug original charge hose and one tank for quick get aways you need to rig up cart of some sort
I do the same thing without all of the plumming. I just use one tank until it gets too low to give a complete fill. Continue using it and get what ever it gives. Then unhook from it and hook up to the full tank for the top off.Richard