xbee via Parallax SIP adapter not passing data to RX of Arduino

I'm trying to test basic communications using XBee (pro s2b) in simple 'AT' mode. Using XCTU's I can pass simple ASCII back and forth, so the XBees are talking to one another fine. Then I connected a Parallax SIP card to to either end and wired them to a pair of Uno's. The Send side of the pair shoots some simple text over the air based on a simple input. I connected the Receive end to an XCTU to ensure the data was transmitting, and the ASCII strings came in just fine - so the Send side of the pair is working great. However, when I put the Receive radio in place and try to capture the RX signal into the Receiving UNO, I get nothing.

I can see all the TX and RX lights flash in a healthy way on the SIP adapter boards. It's as if the Uno's RX isn't listening (put on default pins 0,1 for the serial comms). If I take the XBees out of the equation and wire the RX and TX from both the Uno's directly together the code executes fine and the serial data is passing as it should.

It's as if everything works fine from: Send-UNO > XBee > to other XBee > ...and then the RX signal output from the 2nd XBee isn't 'strong' enough to drive the RX input of the receiving Uno.

Thanks to anyone who has advice to offer.