nicov:
Using 4 would work I guess
I'm having a bit of trouble finding a local source for the low speed full duplex versions but the MAX3082E seems to do what I want so I just need 4 of them then.So if I'm getting this straight this is how I would hook this up:
For the local TX to remote RX:
Local side: DE to Vcc/+5V, DI to TX.
Remote side: RE to Vcc/+5V, RO to RX.
Local A to remote B, local B to remote A. And a resistor between A and B on both sides.
A 0.1 uF condensor between Vcc and +5V, and GND to ground obviously.
Well you really need to show four chips wired to two separate cable pairs. One the local side one chip (for the local's transmit signal) has DE wired to Vcc and RE wired to Vcc, and DI wired to serial tx pin. The other local chip (for the receive signal) has it's DE wired to ground and RE wired to ground and it's RO pin wired to arduino rec pin. The remote has a similar setup as long as you keep in mind the direction of data flow through each seperate cable pair. And yes, four 150 ohm resistors wired from A to B on all four chips. And good on remembering about the benefits of bypass caps at the Vcc pins to ground for all four chips. Does that help? English is a poor method to talk electronics as a schematic drawing is the true language of electronics. 
Lefty