Example 1
David Gilmour and Roger Waters wrote "Wish You Were Here" about former Pink Floyd band member Syd Barrett.
Gilmour and Waters may be well known for not getting along (at all) but in this example they form a compound subject because they jointly performed the action in the sentence; that is, they both wrote a legendary song.
Example 2
Andy's cash, photos, and receipts were all destroyed when he dropped his wallet in the toilet.
In this waterlogged example, cash, photos, and receipts form a compound subject because they're joined by the coordinating conjunction and.
Example 3
My sister and brother-in-law haven't asked me to babysit ever since I "misplaced" my nephew at Target.
We can't say we blame them. We can, however, say that the speaker's sister and brother-in-law form the compound subject in this sentence.