FXS and FXO on the same T1?

I have been running FreePBX 16 and I have a single port T1 card feeding a channel bank (adtran 750) with FXS cards feeding some analog devices on my system. I get my telephone service via SIP, which works fine, except when the internet fails. I have an ATA connected to a different service provider and connected to a different ISP which remains up when the primary ISP goes down. I have added an FXO card into the channel bank to use the analog ATA as a backup trunk when the main SIP provider is unreachable. The issue is, that the T1 configuration in DAHDI does not have an option to make some channels FXS and others FXO, it appears to be an “all or nothing” as far as what I want the T1 to be, all channels have to be FXO or FXS according to the configuration. Is there a way to set up, say, channels 1 thru 16 as FXS and 17 thru 20 as FXO? It’s been a looooong time since I set it up!

Thanks! I’m probably missing something silly but I thought I’d ask!

channels are assigned in /etc/dahdi/system.conf, channels and grouping in /etc/asterisk/chan_dahdi*.conf

Those files have large warnings at the top saying that they need to be managed via GUI and if they are changed, the GUI will overwrite any changes I make.

I have found the dahdi management module not too good unless you have a PRI, make the edits and disable/uninstall the module.