For TXD - If you don't mind if both (all) devices receive data (and ignore stuff not for them, or are already off), you can TXD to all.
Maybe use separate buffers. For RXD, you can either use an O/C OR/NOR gate and only the device TXD'ing will be received.
IF neither of these are suitable, you can use 1 port O/P pin to either select or disable one device. Basically, it's a Network.