I've been interested in dual sources for many years. It started with the (full)
Anthem + K&K (see thread). Because there are no electronics in the (mono) end jack, it's easy enough to wire up a stereo jack and wire the 2 signals in.
Then I tried the
Lyric + K&K and experimented with that in a variety of ways. Not quite as simple since all the electronics are in the end jack, but Baggs was smart enough to design in a stereo jack and solder tabs so it's pretty easy to wire up a 2nd source to the Ring tab.
At some point I looked into the Anthem SL, but since there were no solder tabs (like the Lyric) I gave up. But it's always nagged at me that I thought it was probably possible. So recently I bought another one and figured it out. No solder tabs on the circuit board, but you can wire directly to the jack. The hardest part is sneaking a wire in through everything. I used a twisted pair of single wires because one has to go on top (red) and one has to go on bottom (black). See pictures.
Pictures #1: red signal wire (on bottom) connected to ring jack, black ground (on top) to chassis ground.
Pictures #2: Ring jack, red wire. The other big tab on the left is the switch, don't connect the ground to that or you'll drain your battery in about 2 minutes.
Pictures #3: All done. See previous threads for information on connectors used.