One solution could be using a mixing console converting your signal to mono then routing back the signal to your amp.
In the same category, why don't you convert to mono your output source.
This kind of process is usually really simple with DAWs and depends on your OS but you should find options or programs that help you doing this kind of things
Another one would be to merge your stereo cables together then split them back into two equal cables. This type of practice may need the use of resistors for a better result (don't ask me why)
Here is a really good electronic website (it's a french one so use a translator :) )
Left ________ _________ Output as two mono signals
\ Merge /
Right ________/ \_________