Based on experience shared by other users, you'd send it to both. Even if the spouse uses the same email address as the taxpayer, make sure you have the spouse's email address entered.
Still an AllStar
So when the spouse has an email entered E-Sig will send 2 emails to the same email address (many Spouses share an email)? One for each spouse? That would be great! Does it use one or two E-Sig Licenses?
Yes, it will send two emails to the one address, one email for each spouse. If the client has their email reader set to "thread" emails, they do thread on top of each other, so the client may not immediately recognize there are actually two.
Each return uses one e-signature instance, for every scenario I've encountered. Single filers, joint filers, spouses share an email, spouses have separate e-mails, package has to be voided and re-issued.