Goldenzrule is correct in that you can put a macro on those buttons.

However, keep in mind that by creating a macro in which you have described - left button press resulting in a "Left" command for both the Comcast box and your receiver (which I assume is your AVR, right?) - would be a clunky solution, since the commands will not be occurring simultaneously. It would be one command right after the other.

This would present a delay when you are actually trying to navigate the guide on your Comcast box, for example. If you wanted to scroll to left (or to the right - if you put the same macro for the right button) then you would have to wait for the macro to complete before the button would be able to respond to your next button command. A slight inconvenience, which you can easily avoid by creating a "Press-and-Hold" Macro.

For instance, while still in your Watch TV Activity, a single, momentary button press of the "Left" would control your Comcast box (and only the Comcast box) as it normally would. You could then also program into the remote to set a "Hold" time on the same button to perform a different action.

For example, if you hold down the "Left" button continuously for two seconds, then the "Left" command for the Comcast box would be ignored and by-passed. Instead, a command for "Left" would only be sent to your receiver.

The MX-890 is fairly new. So, I haven't had a chance to play with it yet. But, if it's anything like programming other remotes in the MX line, you can program any navigation hard button with a press-and-hold macro either directly or via a short-cut to a macro that was assigned to any particular soft button "hidden" on another page, for instance.

Hope this helps, but why not just create an HDMI 1 or HDMI 2 button instead? It would seem to be a much easier solution.