Using just one portal, you can get the results you want, but not with the data model that you describe. The key phrase that I am focusing on:
items which are similar, but have some differences
What you need is a single unified table of these items. There are two basic ways that you can handle the "some differences". With all of the data in one table, you can add a field with a value that identifies whether the record is part of the group you currently have in R1 or is in the group you currently have in R2. To handle the "differences", you can either use fields in this table that are empty for R1 record but not for R2 (and vice versa) or you can link in some "detail tables" that only have the fields needed to handle those details that differ between R1 and R2.
But you can also set up a portal to R1 and Portal to R2 on different panels of a Tab control or Slider and then just click to change panels to see either R1 or R2 records.
That outlines 3 different possible approaches that you can explore. I can't recommend one option over the other from the information currently found in this thread.