I actually thought about this yesterday and concluded that the only possible way to do this in standalone mode would be to assign both sounds to pads and then from the main menu enter sampler mode, select the main outputs (in mono or stereo, depending what you need), hit record and then trigger the sounds using the pads. That way both separate samples would be joined in a third sample.
Granted, there would be a silence between them, but you could fix that in sample edit mode.