My opinion is that any time you have a two stage torch and are running two concentrators, it is better to wye the concentrators together to feed the torch. This is because if you are running both fires, and are not using all 10 LPM of a machine on the centerfire, it can go to the outerfire to help feed those jets. If you are running a low flame on the centerfire only, you may be able to keep the outerfire oxygen closed - just make sure that both concentrators are able to put out some oxygen.
If you have on machine running the centerfire and one machine running the outerfire, then on your particular concentrators, you will have to crack open the outerfire to allow for some flow.
Opening the outfire oxygen while running the centerfire can help stabilize the centerfire flame, and can help make it a little more aggressive, but that isn't always something you want to do. It's nice to have the option to run it open or not.
As for a holding tank, you don't have to have one, but it does help when you want to run the torch more powerfully for short bursts (when you empty the tank, you will be back to 20 LPM).
Oh, and while you are new and just getting started using the centerfire, you can use just one machine. You don't have to use both.
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Kimberly
working glass since 1990 - melting it on a torch since 2002
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