Use the lav on one discrete channel and record ambient sound with the other channel using the internal mic. You can select just the lav when editing but you will have backup from the built in mic which can help cover up the rustling of clothing against the mic or wire or any other problem which can occur when recording sound.
Use CH1 SELECT in the middle position (Input 1-) and connect the lav to the top (CH 1) xlr.
Use CH2 SELECT in the top position to record from the internal mic onto channel 2.
It never hurts to have extra audio tracks which can sure come in handy in the event of a problem.
By moving the CH1 SELECT and the CH2 SELECT switches off the top INT (L) and INT (R) positions you cut off the internal mic and record from the xlr inputs on the front of the camera. If nothing is plugged into them you will get nothing.
You can plug the lav into the bottom (CH 2) xlr and set CH1 SELECT in it's lower position (Input 2-) and also set CH2 SELECT in it's lower position (Input 2) and that way get your lav on both channels 1&2 but that is putting all your eggs in one basket. You are always better to record optional sound tracks just for backup.
The best scenario would be to use the lav on CH1 and also a short shotgun aimed at the speaker for CH2 as backup. If you don't have a short shotgun the internal mic is already there, takes no effort and is better than nothing for your backup.
"thou can not stir a flower without crumbling a star" ......Henry Wadsworth Longfellow
On everything (other than DV25) the internal mic is always being recorded. If you have external mics in 1 and 2 the internal moves to channels 3 and 4. So it is always there. Not a real big deal, I normal disable those channels before importing but if I need it for any reason it is there.
One trip I use if I only have one external mic is to plug that into Mic 2 and then set channel 1 and 2 both on Mic 2. Then I change the input levels a little lower on one. This gives me a safety if the levels goes to high on the one.