You can use multiple grounding products at the same time, but for safety sake you should only plug them into one grounding source, either a grounded outlet or a ground rod. Not one to the outlet and the other to a ground rod. The reason is to avoid the risk of a shock in case of a surge due to a power outage or a thunderstorm.
Here’s the situation: Your house/building electrical system is connected to, and stabilized by, a ground rod in the Earth, usually located under the electrical panel. A loop wire (also known as the ground wire), runs through your system, and you connect to it whenever you insert your Earthing cord into the third hole (the ground port) of the outlet. If you plug your Earthing sheet, for instance, into the outlet, you are grounded via the house ground rod.
Let’s say you also want to sleep with a patch or a band. You have an Earthing ground rod and you are thinking about using it now, and connecting the patch or band cord to the attached cord of the ground rod while at the same time you have the sheet connected to the outlet.
Don’t! It’s not a good idea.
If you were to do it, you would be creating what’s called a “ground loop,” meaning a closed circuit − a closed loop with the ground closing the loop. When you place an Earthing ground rod into the soil, usually it is not close to the house ground rod. There are differences in size, depth, and soil conditions between the two rods that create an electrical resistance between the two rod locations. This means a different “electric potential,” that is, level of electrical energy, and the difference would generate a current flow in the closed circuit of which you are a part. So the current flows through you.
In general, you wouldn’t feel anything unusual and you wouldn’t be harming yourself. But there would be risk from a shock, for instance, when a power surge or a thunderstorm occur. In that case, the difference in electric potential can get very high, reaching even several hundred or even thousands of volts, and create a very strong current enough for you to feel as a mild shock.
All Earthing product cords have built-in resistors that protect you. If it were not for the presence of the resistor, the shock you feel would be much larger.
The bottom line: If you use more than one Earthing product at one time, connect them to the same grounding source, either to a grounded outlet or to the ground rod cord. Not one to the outlet and the other to the ground rod.
You can certainly plug two single Earthing cords into the two receptacles of a standard grounded outlet or even into two different outlets. Or you can use an Earthing splitter cord with two terminals into which you can plug two Earthing product cords.
It should be noted that newer houses have ground rods under the house at several points below the pathway of the ground wire, resulting in an improved grounding system. Older grounding systems may have wires with a volt or two, because of the length of the wire and a slight build-up of resistance. Electro-hypersensitive people may feel even that low voltage, and are advised in that case to connect their Earthing product to a dedicated Earthing ground rod planted in the soil outside an adjacent window, if that is feasible).