The DR-100 manual says you can record from one source
(internal mics, XLR external mics, line input) at a time.
You select the source from the menu, and this turns off
the other sources.
There is an "overdub" mode that lets you add a live feed
to a recorded file, creating a new file that is the combination
of both. Useful for adding vocals to a pre-recorded guitar,
or singing harmony with yourself, I guess. The DR-1 includes
the overdub feature, but the DR-07 does not.
Flintstone