Hi Robert,
I think the two TX-pins are fighting each other. When neither are trying to send they are both idle at high (or low, I can't remember). Then all of a sudden one is trying to send a bit pulling the pin low (or high) which effectively shorts the output thru the other slaves TX-pin. It doesn't matter that the second slave isn't actually sending anything yet, its TX-line is still pulled to the idle state of the line so when the other slave is trying to send its output gets shorted.

What you could try is to "isolate" each slave from the masters RX-line with a 1k resistor. I'd probably remove the pullup on the masters RX-line as well otherwise it'll act as a voltage divider when the slave pulls the line low thru its "isolating resistor".

/Henrik.