I'm sure this one's been asked before, but can Valen take his action after the first shot from TLT or are both TLT shots considered one attack? Thanks

They're separate attacks, Valen Rudor will trigger after the first one before the second one happens, and after the second one. If he moves out of range between them, then the second attack won't happen at all. (FAQ v4.1.1, pg. 10, Valen Rudor)