I sign my documents using my digital signature, but when my wife tried to digitally sign a document (not the same document, one of her own), it popped up with my signature and didn't offer a choice to use that one or create a new one. I can't find anywhere that will allow for a second user's digital signature. This is NOT the same as having multiple digital signatures on the same document, just one person's signature on a document but giving that person an opportunity to create a separate digital signature for themselves, rather than the main user.
Wow, you're still using Acrobat 8!
In order for your wife to be able to sign with her own digital ID you need to add her Digital ID to the digital IDs. I have not used Acrobat 8 in years and do not remember the UI that does that (the UI has changed form version to version). Once her Digital ID is installed in the Acrobat Digital IDs store, it will show up in the drop-down list of installed Digital IDs. You will still see your certificate in the "Sign" dialog ("Sing As" field, although it may have a different label in Acrobat 8, I do not remember)) when it pops up, because it shows the certificate used in the last signature signing. When you click on this field or an arrow to the right of it, the drop-down list of all certificates shows up and you can select any one you want. Then the selected one will show up in the "Sign As" filed. It will also show up when you try to sign the next signature (in the same or different PDF).