My first recommendation is to make sure the H9s’ routing modes are set to NORMAL in the system settings. You can do this from the front panel by holding down the right ftsw and Hotknob button simultaneously for two seconds. When the display blinks, turn the big knob until you see UTILS. Press the big knob. Turn until you see ROUTE. Press, set to Normal on both units. Exit by pressing the same buttons for 2 sec.
Next, make sure you connect a cable from the effects send of your amp to H9-1’s Input 1, then output 1 to H9-2’s Input 1. Its Output 1 should go back to your amp’s effects return. Because they are connected in series, you’ll have to exploring the Mix parameter values and reach a balance that works for you in whatever combination your using. Let me know if you have questions.