The dual element setups run both heaters at the same time. In the past, a lot of people would figure how much wattage they needed (3-5w per gallon is the general guideline), and then split that between two heaters. If you need 300w, get two 150w heaters. Need 600w? Get a pair of 300w heaters.
The line of thinking behind that was that if one heater failed, the other would continue working. Or, if one heater stuck on, it would take longer (because of the "half wattage") to heat your tank to dangerous levels, giving you more time to notice and correct the issue.
Running two smaller heaters might be a benefit for cramped spaces as well. Smaller wattage heaters are typically physically smaller as well. It might be easier to fit the smaller wattage, shorter heaters in a sump compartment or the back of an AiO tank than a single longer heater.
With the way Helio works and its built-in failsafes, I'm not sure the dual element thing is as important as it used to be, but there is the option. I have a couple tanks running them now, and I like them a lot. One tank needed 100w, so I run a single 100w element (switched from an Inkbird controller with dual 50w traditional heaters). Another tank needed 300w, so I run a dual element setup (100w + 200w) on that tank.
