XBee-Pro S1 Serial communication not working with modpoll

I have 2 XBee Pro S1 antennas and 2 boards(Parallax USB XBee Adapter). I successfully tried connecting both in Coordinator-End Point and Coordinator-Coordinator settings.

I was able to send console log of both antennas to each other.

I’m facing an issue while I’m trying to use modsim and modpoll from one antenna to other. It says timeout error.

Someone is claiming to have done this by shorting some pins from the XBee board. But I couldn’t find anything regarding this anywhere. Could anyone help me with this? Is it necessary to short wires?