The OEM remote for that dual IQ-1252 light dimmer is sold separately, but it can be taught any button from any other OEM remote. Since you don't have the optional OEM remote, then choose 2 spare buttons (one for each sensor) from 1 or 2 OEM remote(s). Make sure the chosen buttons are really spare (won't control anything else) and the command is repeated when the button is held down. After you have successfully programmed the IQ-1252 light dimmer with these 2 buttons, now your need to teach these 2 buttons to the Harmony IQ-1252 device. However, these is a catch. Harmony will learn these buttons as single action (not continuous when the button is held down). You will need to call Support to change the internal action flag from "single" to "continuous". Note: I am not sure of this (never tried it) but the Harmony learning tool may set this internal flag to continuous, if the command name starts with "Volume..." or "Channel..." and is initially mapped to a hard key.

You can add the light dimmer device to all activities in one go by using the special option "Select the Activities that use this device" under "Device > Settings". It's under "Device > More Options" for the website interface. This special option is only available to Home Automation device types (Light Controller, Climate Control, Home Appliance). However, you will need to use the "Customize Buttons" for each activity to map the light dimmer commands to the desired activity keys. The non-selective automatic command propagation feature was removed a long time ago.