Figured it out:
- Log into My T-Mobile
- Select Profile
- Select the Edit link next to Customer Information
- On T-Mobile ID, select the Edit link next to Phone Numbers. Add the second T-Mobile number and follow the directions.
After doing the above, I was unable to access either account through My T-Mobile.
A T-Mobile representative called me back after an online chat and was told that only one prepaid number can be attached to My T-Mobile at a time, it doesn't allow for multiple numbers. The rep. was able to delete it from the original profile and am able to access it separately. After the second number was deleted, I was able to access the original account again.