I just thought I'd clarify that while the original Shoebox compressor is often considered to be used to "boost" the pressure from a shop compressor (about 100 psi or so) up to PCP pressure levels, it is in fact a compressor and not a booster pump.
Booster pumps use the energy of compressed air to drive a cylinder that raises the pressure of a portion of the original air charge up to much higher pressures. As such, they end up venting a larger portion of the original compressed air back to the atmosphere, with only a small portion making it to the final high pressure state.
But the Shoebox compressor used two stages of additional compression to "boost" 100% of the air fed to it from the shop compressor to the high pressure state, with no air wasted (other than the small amount vented when done using it). It actually is capable of taking atmospheric air up to high pressures on its own without a shop compressor, but would be extremely slow in doing so - removing the ~8 bar feed air would result in roughly an 8 times longer run time.
The best way to think of how that system worked is as a three stage compressor system, but with the first stage "offloaded" to a device that many of us have and use separately - the shop compressor. Personally, I think it is a great way to go about compressing air to very high levels, in part because the use of the shop compressor allows us to remove ~90% of the water vapor from the air stream before we even begin, and to provide easy access to the air after the first stage for thorough drying with desiccants before further compression.
Tying back to the intent of the original post, these compressors can also be run in parallel to speed up filling. I have two (one purchased as a backup in case my original 10 year old one ever dies) and have tried it, but I generally don't do it as I find no need for the added fill rate.
I've been thinking about getting a little shoebox like the Orcair as a backup / travel compressor.
boosters