@agradisher - my two cents:
Dataverse does not support a one-to-one relationship cardinality, so some more context on your business scenario would have helped here. I would question the purpose of building a one-to-one relationship as opposed to joining both tables as one table. This will far more performant and will simplify your App significantly.
If there is a true business need to create a one-to-one relationship, and you only need to View the linked data as one table rather than create Table C, you can leverage a Canvas App or Model Driven App.
Use a Canvas App:
1. In Table B, create a LookUp column to Table A.
2. Create a View in Table B and include the related Column. In the View builder, filter that column to "Contains data".
3. Create a Canvas App
4. Add Gallery control and link the Items property to Table A (GalleryA)
5. Add another Gallery control and link the Items property to Table B (GalleryB)
6. Add another Gallery control and link the Items property to Table B and the View you created (GalleryC)
7. Add an EditForm control and link the DataSource property to Table B
8. On the Item property of the EditForm control, enter:
GalleryTableB.Selected
9. Ensure the LookUp column is included as a DataCard in the EditForm
10. Add logic on your Save/Submit button to ensure the Primary Column in Table does not exist more than once in Table B (CountRows is delegable with Dataverse). E.g.
CountRows(
Filter(
'Table B',
'Your LookUp Column'.'Table A Unique Identifier Column' = DataCardValue1.Selected.'Table A Unique Identifier Column'
)
)
You can then add Label controls in Gallery C to return data from both Table A and Table B. If you do need to create a separate table, you can leverage a combination of this approach together with the suggestion offered by @FLMike.
I would start with this first, prior to doing anything more complex like using Checkbox controls.
Use a Model Driven App
You can broadly achieve the same functionality as above in a Model Driven App by leveraging a real time workflow.
In Table A, create a LookUp column to create a one-to-many relationship from Table A (one) to Table B (many).
In Table B, create another LookUp column to create a many-to-one relationship from Table B (many) to Table A (one).
To prevent a record from being related to another record if a relationship has already been established, you can leverage a real-time workflow:
https://learn.microsoft.com/en-us/power-apps/maker/data-platform/configure-workflow-steps
https://ideas.powerapps.com/d365community/idea/0efbff56-caad-42f6-94cf-79d6794b443b