Differences max485 and max13487
This is probably too late to do any good, but I successfully used these chips a decade ago, talking only to each other (as you're attempting to do here).  The only difference between my setup and yours is that I did set the RE_ line low, instead of high.  From my reading of the data sheet, this causes the receiver (but not the transmitter) to be permanently enabled, rather than counting on the internal state machine to enable it.  That project had numerous other problems, but I was happy to find that these worked first try, with no tinkering.

Looking back at my schematics for that project, I see that I didn't put any pull-up/pull-down resistors on the modules at all, and it still worked.  I agree with your choice of the 20k resistors though (I'm putting 10k ones on my current project).