I don't have my 24:4:2 anymore but I used to use the channel insert jack to run a special insert cable which splits the single insert jack on your mixer to the in and out jacks on your tc helicon. Works really well. Insert cables are not that expensive and available from numerous online retailers such as Zzsounds.com and essentially look like a TRS stereo to mono splitter. You can also buy them as multi-core snakes (as I did) to run multiple inserts out to multiple external hardware devices. If you wanted to process multiple voices at once (say through a chorus effect) you could always run them to an Aux out and track the Aux return but my experience with tc helicon and multiple voices hasn't been too pretty - better off using the Presonus effects channels for that frankly.
