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.
All Earthing product cords have built-in resistors that protect you. .
You can certainly plug two single Earthing cords into the two receptacles of a standard grounded outlet or even into two different outlets.