The Manual transfer switches I have seen are mechanically interlocked or designed so that the sources can not be paralleled. In fact automatic transfer switches

The Manual transfer switches I have seen are mechanically interlocked or designed so that the sources can not be paralleled. In fact automatic transfer switches that are "open transition" have a mechanical interlock as well. Any situation in which it is up to a human to make sure they break-before-make is a grenade waiting to go off. The only thing I've seen close to that is old power plants where the generators are manually synchronized but usually there is still a sync check so the sources can't be connected more than X degrees out of phase, where X is 3 for large hydro units, 10 for standard diesels, and can be as wide as 20 or 30 for diesels at hospitals where they need to get the thing online ASAP