If you want to run two s9s on a single circuit you really need it to be 20 amp.
Presuming his wiring and breaker are both rated at 20 amps, that DUAL 15-amp receptacle is fine and code. Each outlet in that receptacle is rated at 15 amps. This is exactly what I do in my data center (except I used nice brown ones because they looked different, making it easy to tell, and I found them cheaper at Platt).