@Sean_Middlehurst We currently cannot combine datasets between the two experiences. This is because the new experience offers advanced data preparation features like Append, Aggregate, Pivot, Divergence etc. that are not available in the legacy experience.
Our recommendation is to Switch back to legacy experience when you need to use a parent dataset from the legacy experience.
Addressing your concern, we will retain both the experiences for the near future. We plan to upgrade datasets created in the legacy experience to the new experience which will then solve this requirement.
The new experience has been launched to help customers with advanced data prep use cases (when there is no dependency on previously created datasets in legacy). But we hear your request to combine datasets between experiences and we have a plan to address it in future.