In my experience, I was only sending from the Esplora to an Uno, the Uno wasn't sending info back but there was nothing in the project that prevented doing that. RX and TX can be on the Tinkerkit outputs (which can be set as inputs) - hey, that's a thought - try PinMode settings for the two Tinkerkits, one for output, one for input (for xbee transmit and receive).