Pams requirements are very specific and the problem only exists because she goes amp → DAI to add other instruments/drum machine → amp again because the speakers of the combo amp should be used for other instruments and the drum machine.
It would be perfectly fine if she just went for bass directly into DAI. Then it would work with just one so I doubt you need another one.
So to sum up: A second DAI would solve Pams issues but since you found a setup you are happy with there aren’t any issues to solve. Don’t give in to the dark G.A.S. of buying things you won’t even use! 
Maybe we should really start that thread of strategies to convince the better half
Top 3:
- A friend just had it lying around and didn’t use it. (Should be used gear)
- It was a bargain! I had to strike! Otherwise I would spend double the amount in a year!
- “Honey! I bought two basses!” “What the hell?” “Just kidding - it’s just one.”