You need to get them both a few really big kong toys, then stuff them with everything they love (just remember to reduce their meal food to account for this or they'll end up with a weight problem). Stuff them in such a way that it is not easy to get at the food or treats but not impossible either. Peanut butter can be added as well. This should keep them occupied while you are away, especially if they are food driven.

There are also some good books out their on dog behaviour that deal with digging problems.