I have to agree about making sure one dog is trained. However, the other dog can go backwards on you.
My bulldog NEVER dug a hole (mostly because he was too busy sleeping) until I got my Malamute puppy. I've seen numerous holes in the yard and assumed they were from the puppy... hmm
I snuck home one day and took a peek out the back just in time to see my bulldog digging for gold while my malamute looked on with interest. I'm pretty sure that the Malamute instilled the interest in digging but he's not the only one who does it now.
There are lots of ways it could go. It could be a good idea or a bad idea. I don't suppose you can ever really be sure, you just have to be prepared to take what you get.