poll - client extension letter. Are you requiring clients to authorize an extension...

I think this comes under the category of "If your mother tells you she loves you, get it in writing."  There are certainly cases where a written confirmation for CYA purposes is advisable, like when a divorce is pending and you really shouldn't be sure if either spouse is going to remain your client.  And then there are cases where you have known the clients for 20 years, they have always received hefty refunds, and a phone call or voicemail message confirms that nothing was different in 2021.  

Does the AICPA recommend seeking and then reviewing the authorization letters for free?  Or do they recommend adding it to the billing when the return is done?  Is their position totally uninfluenced by the prospect of additional compensation available for dues-paying members?  I can't say, I'm in no position to examine other people's conscience.  

If the spouse isn’t responding, how are you going to get that second signature?  I would go with the suggestion of letting them file their own extension.  Life is too short to get caught in the middle of feuding spouses.

As general practice, no I don't get advanced authorization for $0 extensions.

For your situation, I would probably do a "If I don't hear otherwise from you by Monday at 10a, I'm filing a joint extension" to the nonresponding spouse. You can create a MFS return and only extend the responding spouse.  I'll admit I don't know how that plays out if they later file jointly and owe, if the extension covers all/half/none of the late filing penalty.  But I think that course creates reasonable cause to at least request abatement of half the late filing penalties if the IRS does apply them all.

I do get advanced authorization for auto-debits of projected balances due with extensions.  Not the formal 8879-EXT, but just written email correspondence.

For at least the responding spouse (and hopefully eventually the other one), I'd get 8821's signed so you can pulled wage & income transcripts in a month or two and know that you did get all documents. Given the turmoil in their lives, they are prone to miss stuff.

I'm sorry you're stuck in the middle on this.