[tag1] or [tag2]
[tag1] [or] [tag2]
The or are inclusive.
If you want tag1 yet not also tag2:
As you certainly remarked just giving several tags is taken as 'and'.
One can combine these operators to get more complex queries but for too complex queries SEDE is likely better.
For some more details see How do I search for questions with (or without) specific tags?