My pomapoo who got parvo has been hospitalized now for 4 days and the vet says he can probably come home soon because he is eating now and drinking and more alert, but we have a Shihtzu/Pomeranian at home who is 5 years old and they said that he'll still be contagious for another 3weeks, so how would we keep my Shihtzu from getting the parvo when we bring the pomapoo home from the vet?

Have you other dog at a sitters. They can have no contact, should not even be in the same area.

Has your other dog been vaccinated?

If your older dog has been properly vaccinated, he should be safe, however there is no 100% protection with any vaccine. So this would be a good thing to discuss with your vet before you take the pup home, if this is your regular vet, they should have a record of his vaccine history.
If they did not do it, request that your pup have a bath before it goes home the virus can be in her fur and transfer to your clothes, etc, etc.

Ya my Shihtzu has had all of her puppy vaccines but she hasn't had any boosters.

I called my sitter and some kennels and boarding and none of them will take my Shihtzu because she's a carrier for the parvo.