Never try to connect two amps to the same speaker. This is the best way of generating the most smoke and most expensive repairs I know of. Amplifiers which have been sitting unused for many months may have their power supply electrolytics deformed to a lower voltage than required for proper operation. Bringing the amplifier slowly up to 120 volts with a variac is a good idea. Amplifiers which have been unused for years usually have dried out electrolytics.
Electrolytic capacitors have a useful life of about ten years.
Any old tube amp with its original electrolytic capacitors should have those power supply caps changed out by your tech promptly if you intend to plug the amp in and use it. If you are a collector of amplifiers and are never going to plug the amp in and use it, then you may forego this vital maintenance. While collectors want to see all original components in an old amplifier, players should have the proper maintenance performed on an old amp before any appreciable use is given to it. This is because old electrolytics may seem to be performing sufficiently, but may be leaking current and imitating a resistor.