Once you have an audio interface for your iPad (the iRigProDuo for example) that takes care of 2 audio channels in and 2 out of your iPad. You can do a lot of mixing and routing on the iPad using various apps and your not limited to 2 channels within these. For example Audiobus and AUM are two very good apps for routing audio on the iPad. Quantiloop has it’s own audio routing function as well: it is compatible with Audiobus and AUM and can take the audio from up to 4 channels as input and I think allows for 8 output channels (the 4 loops and additional routing of the original inputs or the metronome etc. - not too sure of all the details). So if, for example, if you had audio from SampleTank generated by a midi piano (linked to the iPad via bluetooth say) you could route that into one of the inputs on quantiloop, record a loop with it, and route the output of that loop to an output channel from quantiloop. At the same time you could route the BB output as an input to quantiloop on another channel and send it straight out to an output channel.
Whatever combo you organise internally, you will typically send the whole lot to the two output channels on the audio interface (iRig Pro Duo) and then to speakers or a mixer etc.