How much experience do you have with electricity?

It's pretty simple to do,

but it can be a pain.

Install the gfi using the existing wires and then come off the bottom side of the gfi to protect the standard receptacle. That way both are protected. You could have more wires in the box than just one, b/c when the house was built they may be using that gfi to protect others in the bathroom or else where. Not sure without seeing it.

the combined calculated load at any given time on the circuit.

If you decide to do it, make sure the power to the recept is turned off, electricity never forgets.

Personally I always discourage people doing this . Hopes this helps.

I have a gfci protected plug in the bathroom and I need to add another gfci with it, double gang I believe it's called. I don't know how to do it exactly. Any help here on the tff?

It's pretty simple to do, more than likely you have a single gang box behind the gfi. You can get the the single gang box out of the wall but it can be a pain. I generally just cut the nails with a key hole saw using a metal blade. Get a 2-gang cut in box and a set of clips to support the box. Install the gfi using the existing wires and then come off the bottom side of the gfi to protect the standard receptacle. That way both are protected. You could have more wires in the box than just one, b/c when the house was built they may be using that gfi to protect others in the bathroom or else where. Not sure without seeing it.

Problem you run into with bathrooms is what do you need all them recepts for. The conductors are prob. only rated for 15 amps and if your wife wants to plug in a hair dryer, curling iron, fan, iphone, flat iron, radio. Then pop there goes the breaker. Hair dryers pull alot of amps. Just b/c you have 4 outlets now doesn't always mean you can run 4 devices. You have to look at the combined calculated load at any given time on the circuit.

If you decide to do it, make sure the power to the recept is turned off, electricity never forgets.

Personally I always discourage people doing this b/c of the water present in bathrooms and the load they think they can put on the recepts. Weather you have 1 recept or 25 you still only have 15 amps dedicated for that circuit. Hopes this helps.

I think it would be a lot simpler to just get a 3 to 1. They work fine unless you are plugging in things that have more than just a standard plug. Unless you run another circuit adding 2 more outlets will not really gain you anything.