If you're referring to e-Signature, you need to enter the email addresses of both the taxpayer and spouse.  In case they share the same email address, you will still enter the same email address for both.

Hi,

Thanks for replying.  But I was asking specifically about the "Client Letter" not e-signatures for e-filing.  I would like to obtain a signature from each client, not just one Taxpayer...as they both pay taxes and both pay my invoice.  When I send the Client Letter via Link, I can only send to one email address.

Ah, I missed the part this is about engagement letter.

First of all, you should be aware that the engagement letter you send from Link will not have e-signature.  All you will get is an acknowledgement that your client to whom the email is sent agrees to your terms of your EL.  Many find that inadequate and use other means to procure proper client signature, especially for those with joint returns.  And, no, you can't add another email address for the spouse to acknowledge your EL.

PTO has added a feature to allow e-signature on documents other than F.8879.  However, you can only use that feature when you send the F.8879 to your client by including those other documents in the same envelop and, of course, that wouldn't be the right time for signing engagement letters.  Maybe that will change some day but it is what it is now, AFAIK.