On the H9 that you want to be the master, i.e. the one that you're going to tap temp on, you'll want to turn MIDI Transmit to XMT and you'll want to turn MIDI CLK OUT to on. On the H9 that you want to be the slave, you'll want to turn MIDI CLK IN to on.
The H9 owner's manual explains how to change these settings: http://www.eventide.com/~/media/Files/Audio%20Support/H9/Everyone/H9UserGuide18.ashx
Once you set this up if you want to test and make sure it's working, connect a MIDI cable from the output of the master to the input of the slave, load up two presets, make sure tempo mode is enabled on both (pressing the right footswitch and the preset switch toggles tempo mode on and off), long press the right footswitch of the master pedal so you can tap in the tempo, and start tapping in the tempo. You should see the tempo value displayed on the screens of both stompboxes.