Being included with Companion Model all you need to do to slave the two units to use the 26.6 AC AMPS total output is connect the sync cable like shown here
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I have the EU2000I, both the req and companion. Only reason is to run the A/C on the 5er. Depends on your A/C. I understand if you have the smaller Polar 9000 or whatever the make/model is you do not need both generators, but it is sure nice to have the power when you need it for what ever reason. Like 4x4 drive, I do not have it, but boy I sure was stupid not go get it. I will never buy another truck without it.