Hi Johnathan,
Welcome to Apple Support Communities. We understand that you're having an issue with adding a second custom signature to your Mail app. We'd like to help.
Have you created more than one email signatures in your Settings? If not, let's have you start off with that: Create and use email signatures in Mail on Mac - Apple Support
Create signatures
1. In the Mail app 
on your Mac, choose Mail > Preferences, then click Signatures.
2. In the left column, select the email account where you want to use the signature.
If All Signatures is selected when you create a signature, you’ll need to drag the new signature to an email account before you can use it.
3. Click the Add button 
below the middle column.
4. In the middle column, type a name for the signature.
The name appears in the Signature pop-up menu when you write a message.
5. In the right column (the preview), create your signature.
You can use the Edit or Format menus in Mail to change the font or layout, turn text into links, or check spelling. To add an image, drag it into the preview area.
If you want your signature to appear after the original text of messages you reply to or forward, deselect the “Place signature above quoted text” checkbox.
Then, let us know what happens when you confirm a second signature or create one.
Take care!
