You really have several options how to do this and each option has it's pluses and minuses.
If you have an extra board
sitting around and lots of 1/4" TRS
(Stereo) patch cables, you can plug
cables half way into the insert plug
for each channel
on your primary (live) board and run that into the line
in on each channel
of your recording board, and then mix down your recording on that board and record the main outputs. This gives you the benefit of big faders for each channel
for your recording but has the downside of being twice as many faders you have to worry about (If you make an adjustment on one, you'll probably need to make the same one on the recording side).
The other option, is to take a post fader
AUX (or two if you want stereo) and record the AUX output(s). This way you set an initial level
that will sound good in the recording (IE: the ratio of guitar to voice will probably be higher in your auxes b/c the guitars probably carry abit live) then you only really have to worry about making adjustments to the master faders and the recording will mimic it.
If you have an extra person who can run the live board (or the recording board for that matter) you are probably better off going with the first option, if your short handed, the second may be better.
There's not necessarily a huge point
in running Auxes to another board unless you really want to have big faders, but with auxes you'll probably have to do most of your recording mixing with the per-channel aux knobs anyway. If you want two boards, and you have the cables, do each channel
using the insert... and just be sure not to bump those halfway plugged in cables during the show!