That adapter should do what you want, provided;
- the "standard" outlet is not a GFCI protected outlet (will trip a GFCI)
- the two A/Cs are wired to the two different hot legs in your distribution panel
- the standard outlet is a 20 amp outlet (15 amp probably not enough)
- if the 20 amp connection ends up going to one of your hot buses that also is connected to some other high current devices, it will probably trip the breaker when operating that A/C and some other device.
You can get up to 50 amps, but not the 100 amps capability you get from true 50 amp service.
