How can you merge 2 channels of an amp?

I have a Crate gt212 and what i want to do is merge the solo channel with the rhythm channel. the solo is basically the high gain channel and the rhythm is on the overdrive. my question now is... is there any way for me to merge the 2 channels to get an in between sound? in terms of circuitry, how may i be able to achieve this? I've always been curious since Adam Jones of Tool has his marshall's 2 drives modified together.thanks...