“Opening ports” on a router is kind of a misnomer. You forward one external port to an internal port on one of your PC’s. You can only forward each external port once. That’s what you seem to do wrong.
What you should do is
-
forward external port 28967 to port 28967 on PC1
-
forward external port 28968 to port 28967 on PC2
-
on PC2 set the external port to 28968 and leave the internal port set at 28967