As always, having a print to write on would be easier. Also, I do not know functionality; materials or target manufacturing process and these do effect optimal dimensional specifications.
Just for the record I'm not sure how part one fits into part 2 with the four protrusions - must be made of plastic.
As I have already said, my suggestions are being made with minimal knowledge of fit, form and function (FFF).
Part 1:
External diameter assigned Datum A, maybe a cylindricity, circularity or a tight limit tolerance to control form and size.
Option a - The top two tab width features assigned a limit of size then position tolerance relating the tab features back to Datum A. Declare the both tab features as Datum B. Datum B will give you rotation or an angularity reference for the remaining features
Option b - Define one or two (opposed to each other) of the protrusions as Datum B and use position tolerance to locate back to Datum A.
I think the protrusions are a more obvious choice for datum B as they relate to the protrusion cutouts on Part 2. Remember that I have very limited information on FFF. I don't know the relative mating importance of the tab feature though I suspect they matter within the target assembly.
Option a - Position tolerance the four protrusions back to the top two tabs Datum B secondary and external diameter A primary.
Option b - Position tolerance the two tab features back to Primary Datum A and then secondary datum B protrusion or protrusions.
You could use Profile of Surface to locate, orient, size and form control the entirety of the top tab features and either declare the two exclusive top tab features as Datum B or simply relate back in entirety to the protrusions defined as datum B.
PART 1
For Part 1, I recommend datum matching or mirroring relative to part 1. I assume you know that I'm talking about...
Since the cutout feature on Part 2 are likely to be the secondary datum B, relate the four threaded holes (I think I see screws in the threaded holes) back to Primary datum A internal diameter and secondary datum B cutouts.
FYI, your question was very significant for a discussion forum.