Thinking about the order of gas filtering. If you do a serial configuration you will end up having only one pipe with leftover unfiltered gas at the end as it's the waste that remained from the very last filtration. If you do a parallel configuration you will have to plumb the unfiltered output of ever filter box thingy into the input again as they can only separate out 1 (2 if you use 2 filters) gas from the mix and the rest of the mix would go to waste which would still include all the other gases that you'd use. That would mean you would also need a filter for gasses that you don't want to filter them out of the shared input pipe. Serial I think is the most efficient way to filter gas... unless they add a filter thingy with more filtered outputs.