Can I connect two microcontrollers via RS232 separated by a distance of 3.5 inches? Both are on two separate PCBs
If the distance is really  3.5 inches, it is quite absurd to think that when connecting two UARTS together you must use RS-232 tranceivers. You don't need to use RS232 tranceivers and you don't need to use buffering. They can be used of course, but why would anyone connect two MCUs at 3.5 inches via RS232 tranceivers or buffers  unless there is something we don't know.
Also neither TTL or RS232 levels will work unless the boards share common ground, but I suppose ground between boards exist but is not explicitly drawn in your diagrams.