Let me clarify terminology. There seems to be a a misunderstanding. You don’t import ipynb file as a sheet. *.ipynb file in jupyter terms is a single unit of work with its own python environment, which is important. In datalore such a unit is represented by the notebook. Thus it’s incorrect to say
you can only import and use an .ipynb file as the first sheet in a notebook
You convert existing *.ipynb file into a datalore notebook.
Speaking about having multiple *.ipynb files in a single notebook doesn’t make sense because how do you choose python environment for a notebook then? A sheet is just a tool to make your notebook more readable and composable. You might want to have all your imports on the first sheet. On the second one there might be some model training and third sheet might, for instance, contain some visualisation of the results. Organizing your work in such a way could be useful and ease navigation.
Hope that clarifies things.