2 Expansion Tanks

I discovered that I have two water heater expansion tanks, one off the water heater and one under the floor in the crawl space. Do I need to have two expansion tanks? I do have low water pressure could this a be a possible reason why my presurre is low in the house. I only have one water tank in the garage.

You only need one expansion tank, but having one doesn't lower pressure.
Having two is fine.

Do you have a boiler for heat? That second expansion tank may be for the boiler, not on the potable water system. While they look the same, you shouldn't use one for the other - i.e., not all are rated for potable water use.