One way is to use two channels. If you want to display the data from both channels at once, you can create custom MATLAB visualizations and ebmed them in the channel you want to see them on. The visualizations can read data from channel B and display it on channel A, etc.
A less optimal solution, but still possbile, it to pack multiple values into a single field. There are up to 255 characters allowed in each field, so you can put multiple valeus with a seperator. Then you need to write code to deconstruct and plot the values since the ThingSpeak field plots wont work automatically. This code in MATLAB is no too complex, and you can use the AI chat assistant at the playground to help you write it.
An even more convuluded aproach (that I actually use) is to seperate the values in time. You can post the values sequentially (keeping the update rate limit in mind) and then seperate the measurements by time. I have 10 devices that control lights on my driveway, each one posts its battery state to the same channel every few hours, then I seperate them later in code.
