Wonderful, we look forward to hearing what you think about what is there so far and how you'd like to use it! The documentation is a good place to start.
Can you please describe what you want to do in a bit more detail? I'll try to give some information that will answer your question but I would find it helpful to learn more about what you have in mind.
You can currently link as many datasets as you want to any kind of form including a registration form.
If a form registers entities in a specific dataset, it can also attach that same dataset and access its entities. This can be useful to do things like identify whether a given entity was previously registered. Note that currently entities are not created offline so entities you have registered won't be available until they've been submitted, the submissions have been approved, and the device has updated its dataset.
Currently a form submission can only create at most one entity. In the future it will be possible to register multiple entities at once either through a repeat (multiple entities in the same dataset) or not (multiple entities possibly in different datasets).
You may also find some of the discussion and ideas at Entities - get the most recent record from the database useful.