Try to turn the compressor by hand. It should be possible to turn it around. It should turn around smoothly, without any 'hickups' (be sure to completely shut down the air compressor!). Check the isolation of the motor windings.
[pdf] They are powered by two counter-rotating screws that compress air as it moves through the compressor. These units are designed for continuous use, capable of operating 24 hours a day, 7 days a week.
[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] 
