You do not need a mixer. The L3t with HD500 is sufficient. You can connect the HD500 via L6 Link or to the Line inputs on the rear panel of the L3t.
You can experiment with different setups. For instance, with the acoustic setup you could use the L3t side panel mixer for both vocals while running the acoustic guitar through the HD500, or you could route the acoustic guitar into the L3t Channel 1 (enabling you to use the onboard L3t acoustic guitar modeling) and have one of the vocals running through the HD500.
You could even use 2 mics and both acoustic/electric guitars simultaneously by using a dual-path patch in the HD500.
Even with the backing tracks there are options. You could connect them to the cd/mp3 input on the HD500 or to the Line inputs on the rear panel of the L3t if you use L6 Link for HD500 to L3t.
A very flexible setup with no additional mixer.