For sure --- the simplest configuration is a shared database that is accessed both by your provider FHIR app and a traditional patient web or mobile app. Users can collaborate around care plans or whatever by accessing the same records in that database.
If your goal is to use the FHIR server as a read/write store for collaboration, it's a bit tricker, because both the schemas and (more so) permissions for writing arbitrary data using FHIR are pretty limited. But it's not out of the question depending on the scenario, you'd just have to build both provider and patient apps pointing at the same FHIR server(s).
Last, theoretically if the patient is physically present where the provider is using the app, they could collaborate around a single interface --- but that presents a bunch of security and provenance problems unless the provider is entering the data on behalf of the patient, e.g. walking through a survey checklist verbally.
Hard to go deeper than that without more detail on your specific scenario, but hope it helps get you started.
---S
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