When viewing a list on all platforms (Mac, iPad, or iPhone), you can filter by more than one tag.
- Mac: Press and hold Command (⌘), then click your desired tags.
- iPhone: Filter by tag, select one tag, then return the same menu for the ability to check more tags.
The “Using Tags” support page by Cultured Code gave a handy example:
It’s useful […] if you want to see all your 🏷 Work to-dos that are also🏷 Important.
For Todoist, see “Filter for tasks by label“. As for Wunderlist, no joy.
All Narfed Up 
Reblogged this on Dwain Maralack and commented:
The ability to filter by multiple tags will elevate my prioritization process.
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