Alex here, from the Shopify support team.
While the Shopify core tag functionality cannot be currently altered to sort by OR condition, an app such as Power Tools - Filter Menu can help you customize your tag filtering beyond what's possible within the Shopify platform.
In the meantime, I'd be happy to pass this request for more customizable tag sorting along to our developers so they can keep it in mind for future changes to the Shopify platform.
I hope this helps! Please feel free to follow up here with any other questions.
Alex | Shopify Support