well, you could set up ‘cross’ and ‘mtb’ in your profile and track each? Often my mtb rides have sections of road in when I am getting to and from places….and (very) occasionally in summer I will take my roadie down smooth looking bridleways. I log based on the bike I’m on rather than the surface I am riding. Depends what you want to track for your own goals really?
I used to break up my rides but don’t bother now as I don’t want to clutter my or my friends feeds. So morning commute, lunch time ride and evening commute will all be on the same ride, will be something like 90mins riding with an elapsed time of 10hours. Can do that on the garmin, not sure if you use the mobile app.