can a reader thread and a writer thread r/w to same fd

RedHat Linux 7.3

Can program A with 1 thread for reading and 1 thread for writing read and write to the same open socket that is connected to Program B?

Can program B with 1 thread for reading and 1 thread for writing, read and write to the same socket, which is the other end of the connected socket of program A?

I am using select in these programs. They start off fine, then they get into a pattern of blocking for about 35-40 milliseconds.

yes, you can r/w descriptors freely from threads, that type of synchronization is handled by the kernel.