Status of 'Dear Both' as a salutation

It seems quite common (although not universally accepted) to open emails addressed to a large group with "Dear All," (see e.g. this question). Extending this logic, is seems reasonable to open with "Dear Both," is the group you're addressing happens to have exactly two people in it.

It goes without saying that it is not appropriate for formal letters, but I am wondering - just how inappropriate is it? Is it OK in semiformal correspondence (to one's coworkers, students, etc.)? How does it sound to the native spreakers? (i.e. Does it sound/look unnatural?)

Edit: I have tried googling, but unfortunately, this does not produce very satisfactory results. It's mostly forum threads with contrary and unsupported answers (ranging from things like "I've never seen it used" to "surely, it's been widely used even before emails").

Solution 1:

Yes. Unbelievably, it is "reasonable," with closely acquainted addressees.

The process of growing friendship can be seen in the greetings: Dear Signora, Dear Mrs, My Dear (the surname crossed out), Marcello and Munira, Cara, give place to My dearest, Dear Both and in the last one, Dear Child.

^{Sean O'Faolain's Letters to Brazil, 2005, p.172}

Note "growing friendship," "greetings," "give place to" leading on to "Dear Both" – "Both" is not a proper name in the context, by the way.

Though not quite a usage in direct addressing, the phrase "dear both (of 'em)" can be found in Dickens!

Then my dear both of 'em bowed very low and haughty and parted, …

^{Charles Dickens, Christmas Stories, Vol.II, p.53}

Solution 2:

"Dear Both" strikes me as very contrived and awkwardly formal. If you're addressing an email to two co-workers, "John and Susan", seems much more appropriate. If they're friends, "Dear John and Susan" seems much more personal.

Solution 3:

This year I have started seeing a lot more emails addressed to 'Dear Both'. Consensus though with my professional peers is that it is lazy, even a bit rude.

If there are only 2 recipients then writing Dear 'Tim' and 'Sarah' for example is much more polite and takes little more time and effort. A simple greeting of 'Good afternoon' or 'Good Morning' is more also more polite than Dear Both.