Yes, you can use a mixer.
Please note: If the L1®s are less than 20 feet apart and you run any source (e.g. microphone) to both L1®s you run the risk of greater feedback and some odd side effects (hollow or ringy sound, and odd differences in volume in different parts of the room).
More details here: Stereo, Mono, Distributed Systems
Having said all that, it's pretty simple to make the connections.
Run all your inputs to the mixer. Note: we are talking about a non-powered mixer (no power to drive loudspeakers). Line-level outputs only.
Assuming the mixer has LEFT and RIGHT outputs:
Take the LEFT output and run it to one L1® - typically Channel 3 or 4 and set the input level to around 3 (you'll have to do some tweaking).
Take the RIGHT output and run it to the other L1® - typically Channel 3 or 4 and set the input level to around 3 (you'll have to do some more tweaking).
You will probably not need to use the R1 Remote Control.
Ideally on the mixer you would pan each input fully to one side or the other to avoid the problems that can arise if you send an input to both L1®s. But since you are asking how to do exactly that, then for the vocals that you want to hear in both L1®s, set the pan in the middle.
From what you've said, you will get the most benefit from this on large
stage where you have the L1®s beside or in front of you. There will be little benefit (and greater likelihood of unwanted side-effects) on small stage or any situation when there is less than 20 feet between the L1®s.
You won't be using the Presets any longer and although it is possible to work with those, you would need a multiple buss mixer to make that work, and things get considerably more complicated.