You can't really hook two amps into one speaker cabinet, with a single input jack.
You could provide an additional speaker jack in the cabinet, with each jack feeding a single speaker... so the Sonaro would drive one speaker, and the Concert would drive the other.
If you don't want to add another jack... AND both amps have 3 prong plugs, you could replace the speaker mono jack with a stereo jack, and rig up a double mono plug to single stereo plug speaker cable.
The power amp input jack on the concert is to supply the power amp with a line level signal (i.e. already preamplified) from another source... like a different preamp. When this jack is used, the concert's internal preamp is disconnected from the internal power amp. The combination of the preamp output and poweramp input jacks provide for a "effects loop" where line level effects can be plugged into.
