Other apes with gained intelligence?

"Caesar was with another big ape" you mean this guy, Maurice (Orangutan)?

Then this big ape did not gain intelligence from treatment but he knows sign language because he was in circus in the past. At this mark in the movie where Caesar and Maurice were talking in sign language, Caesar asks

00:49:07,361 --> 00:49:10,363 You know sign?

then Maurice replies

00:49:11,449 --> 00:49:14,075 Circus Orangutan.

So, it's pretty obvious that Maurice gained his intelligence from working in Circus.

Apes have been trained in the use of sign language for decades. However, whether they truly understand the signs they use or simply become good at mimicking them to earn some type of reward has been disputed for about as long as they've been trained to use sign language.

Read more about Koko the gorilla for a bit of insight into training apes to use sign language.

From the film's perspective, the sanctuary Caesar is placed in is home to apes used in other various research endeavors, or rescued from owners, just to name a few examples of why they'd be there. The ape that's able to communicate with sign language likely learned it as part of a research program it went through prior to ending up in the sanctuary.