I don't think this is possible without an automation or extension of some kind. It does look like the Record List extension would be a good fit for what you're looking for, though!
I have an idea that doesn't require an automation. In the registrations table, you'll need to add 2 fields: A lookup field that points to the customer's start dateA formula field that computes if the order date/registration date is on or after the Cu...
I don't think it is possible to automatically keep them sorted, but it is possible to use the Batch Update extension to sort the linked records. I believe the linked records are what control the order of the rollup data.
Assuming that the find records step is in the top Actions section and isn't already a Conditional Action, this should be a pretty easy fix if you make all actions following the Find records step conditional. You'll need to add a group of Conditional ...
It sounds like you may be able to get what you need without building a third table.I suggest you create a linked record field on the Items table that will allow you to Item records in table 2 to a particular Client in table 1.Then you can add Lookup ...
