I have written about add a compressor a number of times on different posts and articles. I would consolidate the various writings on one page for the benefit of those wondering if they should add a compressor and as an answer to this specific questioner.
I saw a previous answer that suggested that:
1. both compressors would lead into the same air main, and it might be helpful to have that main lead into another receiver before pulling air to the application
2. that in the lines from each compressor, I would install a check valve that allows outflow only. That way, if one compressor is running behind on recharge, the other one's output won't flow into the other compressor's receiver.
On another web site though I saw a suggestion that the tanks on both compressors should be directly connected. So that confused me somewhat. Perhaps that approach is more applicable to vertical compressors with very large tanks.
Anyway, if the suggested approach in the first answer is safe etc, I'd appreciate a bit more detail on what the most practical way is to connect the air hose from the two compressors into a main line, including preferred location for installing check valves.
Since I currently have quick connect couplings on the Makita, I was thinking that there's probably some kind of T-fitting or manifold that one could jerry rig.
If you simply were adding a tank (or three, or four) to an existing compressor yes, the new tanks would enlarge the reservoir of the one compressor. The result is you would have more pre-compressed air to draw on when running an application.
However, then the original compressor pump would have to run longer to fill the larger reservoir(s). A real concern is the existing compressors duty cycle.
Instead of adding additional compressed air tanks to one compressor, which has the potential for negatively affecting the life of your pump and motor due to the duty cycle issue, you can add another air compressor.
Combining two compressors is a bit more complex than simply adding another air tank because the second compressor, too, has a pressure switch and possibly different cut-in and cut-out pressure settings.
In order to get more compressed air flow at the pressure we need to our air tools, we have two choices. We either upsize the existing compressor to a larger and more expensive model, or we decide to add another, smaller air compressor to the circuit to increase compressed air availability to resolve the flow problem.
At this point, if you are interested in looking, here is a selection of air compressors available from amazon.
If you are running a 10 HP compressor, it will theoretically generate around 40 CFM of compressed air at 90 PSI.
If your air tool requires 80 CFM of compressed air at 90 PSI, then you won't be able to run that air tool very long before the compressor tank(s) are emptied, and the compressor is unable to supply enough air.