Сan I put "tam" at the end of the sentence?
Yes, but then you have to start with the trees and the "there" becomes stressed, you're really pointing with your finger at the trees:
Dva žluté stromy jsou tam! - There are two yellow trees over there!
Is this also acceptable: "Tam jsou dva žluté stromy."
Ano, to je také správně. ;) It's being fixed now.
The system marks this as incorrect: ""Tam jsou dva žluté stromy."
Yes, I added it.