The questioner asked about the safety of adding another air compressor.
If the components used are rated for the pressures of the system, and the person takes care not to open a charged air line or work on an electrical component without ensuring that the power is off, that should handle the safety issue.
The questioner mentions that they have visited a number of websites to garner information. That's good. You want to get diverse advice. It's not my place to comment on the accuracy of information gleaned elsewhere. Here is my concept.
I've shown both lines combined into one at a Tee, and then the single outflow line passing through one compressed air filter to the application.
Since compressed air cools as it travels through air lines, and since it is moist, high-humidity compressed air, water will condense out as it travels to the point of use. Depending on how far the lines run, and how tolerant your end application is for free water (most applications are not) you will want to install additional air filters at the point of use.
Here's information on compressed air filters on this site for you. If you need a source, perhaps compressed air filters amazon can help.
Poly tube will use one kind of fittings, rubber hose will use another, and if the air lines are black pipe (I don't recommend it) or copper pipe (I do recommend copper) then fittings will be different again.
You will need a method of connecting each compressor discharge port to the appropriate check valve, the check valves to the Tee, and the line out to the application to the in-line air filter and the application itself.
Understanding how couplers and connectors work together will help, if you aren't comfortable with this type of fitting. Information about couplers and connectors is here. If you are looking for a source for couplers and connectors, this link is a source.