Your problem is that by connecting the OE pin to +5v, the transmitter is enabled all the time.
So that when one transmitter sends data, the line is loaded by the other (non sending) transmitter and the data is corrupted.
You need to keep the OE (Output Enable) pin low, and only set it high when you want to transmit.
You will also get a problem of both outstations transmitting at the same time.
To overcome this problem, you must "listen" to the line using the receiver in the MAX487 to see that there is no signal on the line before you transmit.
JimB