Simply using another 3 phase motor for a phase converter won't work very well if at all, especially for a high start load like a compressor motor! Three phase converters using another motor are very common and work quite well but they must be paired with a large capacitor bank to work properly, these converters can be quite expensive (one big enough for 7 HP will be around $800 to $1,000) so just to run one compressor it's probably not a good option. Trying to run a high start load by using just a second three phase motor is usually just an exercise in futility and at worst could smoke the compressor motor!
Here's a schematic for a common design,
The capacitors can be eliminated for a simpler design but they only work for light start loads and would be very problematical for something like a compressor which is almost always a very high start load situation.