You can do like I do, just daisy chain them. No modifications needed, and you also have a spare, very unlikely both would fail at the same time (not that a Rumble has ever failed me

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Run a cable from the effect send of combo on the top (= master) to the effect return on the one below (= slave).
Turn on both combos. All the tone, volume and such adjustments you do on the master. The controls on the slave can be in any position.
Works fine with the 500 + 200 and gives you a 490 watt, 2*10 + 1*15 stack. ... ...