I did a bit more looking around. The latest version of Marlin now support three serial ports. Old versions only supported two, so you would have had to give up touch mode on the TFT in order to install a pendant with only two serial ports. The instructions to get the third port working are in this topic.
But, if I’m reading the topic right, V1 maintained versions have not updated to a version that has this feature enabled…but may soon 
.
So if you wanted to explore a pendant now, you could give up touch mode and then re-enabled it when V1 updates their version of the firmware.