Many of those fittings allow a female fitting OR, you can solder the pipe inside of the thing directly. Plumbers generally prefer to solder them in when possible. If yours can do that, I'd remove one, cut the pipe to fit, clean both the pipe and the fittings, then remount them with the pipe in there...then, solder it in. Much more reliable. If you thread the fittings on, then tried to solder the pipe in, you'd likely compromise the threaded connection's sealing, either the tape or pipe dope (neither really likes to get super hot). Why have twice the joints?