As far as I know, no there isn’t. There is always the generic “cardio” activity which you can track with the data fields, timer, distance, pace, speed, heart rate and calories.
If you export the activities as FIT files (originals), you could use Fit File Tools (https://www.fitfiletools.com/) to combine them into one file, then import that back into Garmin Connect.
Depends what exactly you mean, and depends what device you own. Watches supporting the "multisport" feature allow you to combine several different sports into a single "multisport" activity. A typical example is a Triathlon training or a race, but the activity may consist of more parts, and can use diverse other disciplines, not only cycling, running, and swimming.
And if you rather meant merely merging two (or more) activities of the same type into a single activity, then it is possible too. You can export them in the FIT file format (the option "Export Original" in the Activity cogwheel menu in GC Web), and then you can use for example the [File Combiner of the FIT File Tools website], importing the result to GC Web on the page [Import Data]. You could do it also with the TCX file format, merging the files manually with a text editor, but I would not recommend doing it.
As far as I can tell with my Venu SQ, there isn't a multi-sport option on the watch itself. Under the IOS mobile app there is also not a way to create a multi-sport activity even with the "custom" workout as in you can't pick the type of activity you're adding a step for.
