First, check that there isn't a very high air demand, or air leak somewhere. If the capacity of the air compressor is really too low, check the following: Check differential pressure over oil separator. Replace separator when necessary. Check and replace compressed air filters (if installed).
[pdf] We will start our trip through the loop at the same place we will finish: The oil sump. This reservoir is where oil is stored for use in the system. It is usually located in the lower section of a tank-like cylinder.
[pdf] To connect two compressors in parallel, follow these steps: Make sure both compressors have the same voltage and phase. Install a check valve on each compressor’s discharge line to prevent backflow. Install a tee fitting on each compressor’s discharge line. Connect the two tee fittings with a pipe..
To connect two compressors in parallel, follow these steps: Make sure both compressors have the same voltage and phase. Install a check valve on each compressor’s discharge line to prevent backflow. Install a tee fitting on each compressor’s discharge line. Connect the two tee fittings with a pipe..
The Short Answer: The basic process involves setting up a single pressure switch system linking both compressors, using a check valve to prevent air backflow, and using suitable hoses or pipes.
[pdf] 
