Is it possible to add two secp256k1 points to get a "shared key"? As I know from the Elliptic Curve Diffie Hellman (ECDH), you can create a "Shared Key": https://asecuritysite.com/encryption/ecdh2

This will look like this:

dA × dB × G = Shared Key

QB = dB × G

QA = dA × G


But is it possible to get the sum of dA and dB? (Plusing the value of each private key?)

dA + dB × G = ???


How to get a "Shared Key" when adding each private key (plus)?
Yes, QA + QB = (dA + dB) × G, but this isn't used in ECDH as QA + QB can be calculated by third parties.

You can read about this property here.