Elg-3a blaster pistol and the concealment holster both give setback to attempts to find this weapon on someone's person. Do they stack?

I would stack them at my table.

Yes, you can add both. You may be confusing the RAW on Setbacks from the same source not stacking in Combat for Defense, in that case you use the highest.

1 setback for the blaster's profile, 1 setback for the placement/profile of the holster. Sounds fine to me.