From the designer:
Both amps are equal and 100W capable but the supply is only rated at 100W. If you use it for a 2-way speaker this is not a problem since the average power on the tweeter is quite negligible. This was its primary target application, hence the choice to rate the supply only for one full power channel. So when you use it to amplify a stereo pair, consider it a 2x50W amp.