Introducing cats

It is say 5 of having a second cat. I had 2 cats once before and all went great. This time around 1st cat is 11 month old male newest addition is 2 year old male both neutered.

Of course still separate and will remain separate till 2nd cat is vetted by my own vet Monday but i have a question.

There has been no hissing and resident cat was intrigued and vocal but now he is kind of eh and more so avoidant of the door where the other cat is but they both yesterday played separately but with that door open while I sat in between. Any advice or thoughts? Thanks.