It is possible but care would need to be taken to ensure that both parents have been tested for genetic diseases which both breeds can suffer from, such as eye problems.
I think you would be more likely to find a Cavalier King Charles Spaniel crossed with a Shih Tzu, as King Charles Spaniels are much rarer and breeders would be far less likely to allow one of their dogs to be crossed, as they are trying to promote the breed as it is classed as vulnerable by the Kennel Club, whereas the Cavaliers are available in abundance, despite the heart problems which beset them.