You should certainly try it both ways—amps in single outlet vs separate outlets—-and reach your own conclusions. Each system may react a little differently.
In my system, using tube monoblocks, the sound with separate outlets is slightly brighter, slightly leaner, slightly bigger and more spacious, slightly recessed in the mids, and slightly more exciting. With both amps plugged into the same outlet, the tonal balance is more linear, more coherent, but also slightly smaller and less exciting.
In past years with some different tube amps and different speakers, I preferred the sound with separate outlets. With my present setup I hear the same types of differences noted above but overall I now prefer the single outlet.
In my system, using tube monoblocks, the sound with separate outlets is slightly brighter, slightly leaner, slightly bigger and more spacious, slightly recessed in the mids, and slightly more exciting. With both amps plugged into the same outlet, the tonal balance is more linear, more coherent, but also slightly smaller and less exciting.
In past years with some different tube amps and different speakers, I preferred the sound with separate outlets. With my present setup I hear the same types of differences noted above but overall I now prefer the single outlet.