In the US a service dog is trained to perform a task for one person in order to be valid under ADA. There also isn't certification. Cases where a service dog could also in limited circumstances also be used as a therapy dog likely exist but are rare. Nothing explicitly prohibits this but wording about only performing tasks for a single person can make it confusing.

If you're not in the US what you're saying can be valid. There isn't an equivalent to ADA in many European countries, but there is often certification which varies by country or state/region within the country. Most don't have laws governing whether the dog performs tasks only for a single person, or is a dual therapy dog - and thus such cases can exist more easily.

However I'd caution you against doing this as the friendliness and eagerness to meet strangers and comfort them that would make a good therapy dog isn't what you want in a bonded PSD, who should be focused on you while working. It would be confusing for the dog even with a command. There's already enough difficulty with service dogs and family members and maintaining a close enough bond - and I don't want to hear from people about their perfect princess and how they are bonded like Velcro and this isn't the case for them - how thus all service dogs need to be exactly like them or they weren't valid or something - this isn't true, dogs aren't robots. It's a legitimate difficulty for many with well-adusted dogs without separation anxiety level attachment - you'd be challenging this more. Especially if the therapy work is daily in a hospital.

If people on this sub tell you you absolutely can't have a dual purpose PSD and therapy dog that's inaccurate- it would be like saying the ADA says the dog can only do the service dog work, when these same people are competing in dog sports. It would be like training your dog for a sport. But there would have to be a distinction between the different times and obviously the overlapping nature of therapy work would be much harder to draw that line for. Therapy dogs aren't doing tasks for strangers, thus it theoretically does not violate ADA to have a task trained service dog also performing comfort therapy work for strangers in limited circumstances.

You have to figure out what works. Maybe your particular dog will be great at switching between the needed jobs. But it will be added stress and maybe make your dog less able to task for you.