Go to impressed-current. If you didn’t have a water softener, you could get by with aluminum/zinc anodes, but those often don’t work well in softened water. You can try just putting an impressed-current anode in just the powered tank, but you’ll probably have to do both of them.
I’ve dealt with geothermal systems in the past and the water in the tempering tank is usually warm enough to promote the bacteria that cause the odor. Get SKU25s from here: http://www.waterheaterrescue.com/OrderPages/Power-Anode-For-Combos.html and note that the plumbing will have to be changed to use them. Peroxide IS a solution, but a temporary one. If the condition is chronic, the odor will always come back.
People always buy impressed-current anodes for odor problems but another very important reason to have them is that softeners can chew through sacrificial anodes very rapidly and then heaters rust out.