That would be my next step. Block it so that they can see through but not get through the doorway. See how that goes.
Then you can try to open it and see what happens. Usually if there is a conflict, each party will respect territorial boundaries until things improve.
Even if something does go "wrong", all is not lost. They can forgive and forget. Every day is a new day for a cat. It just might take some more time.
When you can get them in the same room, playing with both at the same time is good, like with a wand toy. Each can be focused on the play and sneak a look at the other at the same time. Hard to be afraid of a playing cat.
Hissing and even growling is just cat politics as usual. Just ignore it. It will go away eventually. You will know you are "there" when they don't hiss or growl but just sniff each other.
If the older one is annoyed by the younger you may still get a hiss or growl that says, "back off and leave me alone!" even after several months or years. Not to worry as long as no fur is flying. 
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