have you tried importing the video background and audio into your video editing software and then doing multiple copies of the VIDEO footage on th etimeline so as to be the same duration of the AUDIO file.
Export the VIDEO and AUDIO as seperate streams (AVI) and then open up in Encore and ALT+drag and drop the background video onto the menu and then just drag and drop the AUDIO file onto an empty area of the menu and then set the loop to FOREVER. This is one way around the 15 second max duration that Encore allows..
...Surely I can set two different loops for them, rather them being connected?
To do something like this, you would user Premiere Pro or the NLE of your choice to loop one track or other to match the longest track, then export this as an AVI and set it to the background of the menu.