If I try to play a Shadowcloaked Lurker from trash and I currently have two of them there, does it get the discount from both of them?

No — you get the discount from one copy, not both.

Cost is determined after the card you're playing has already left the trash and gone onto the chain, so it can't count itself. With two copies in trash and one being played, only the remaining copy still in trash applies, giving a discount of [2], not [4]. (FAQ #8430; see Rule 356.3–356.4 on determining total cost.)