Umm..err..you POSITIVE that they are both females? Even if they are, you new little girl just likely is pregnant!! It's very often the case when someone gets rid of a pig, because they keep having babies. Can you post up a picture of her? If she has a pear shape it's good chance she is. If she's pregnant you'll have some fun times ahead and a ton of work, since CC cages have to adjusted to caring for babies.
If it's just sheer overindulgence (
which would be weird if they are both housed together) then just getting them out for some good floor time for a few hours every day would help them burn up some calories fast, See if you can set up your living room or dining room as 'piggie friendly' so that they can get out and get a good run, but have plenty of places to hide. I use baby gates to shut off different parts of the house so that they can have lots of fun. There are hundreds of ideas posted here on ways to do floor time but the important thing is to get her out and moving and grooving.
In addition to that you'll want to read about proper diet and nutrition so that you can make sure you get her off of any extra caloric food as well. (Get off the Kaytee pellets as fast as you can, they have bad chemicals in them) The very
best place to get that information is on Guinea Lynx where they have posted up a wonderful
Guinea Pig Care Manual online. When you get to the page, you'll notice that there are blue links in the text, and also there are links of the left side of the page. Be sure to explore all of them as you need to. Be sure to bookmark that webpage for future reference as well.
Guinea Lynx :: Guinea Pig Care Guide
The other thing I should ask is if she has long hair. Long haired pigs are often all hair under that appearance and a good short haircut will sometimes fix it.
In the meantime, look into getting something other than a pigloo. Personally I hate them, since they only have one door and pigs often argue which leads to tossles inside them. I like to use Super Pet Brand Large sized Grassy Hutches. They can be ordered online or ask your local pet supply shop to order one for you if they don't carry them. Most small local non-pet selling stores are happy to do special orders. I keep a couple in the cage and they use them to go from one to the other, plus they are large enough for adult guinea pigs.
There are also these in several sizes. They are also made by "Super Pet" but sometimes the petstores have different brands and styles.
There are hundreds of other options as well. Including something as simple as a box! Just find a good sized box, cut a door into it, then FLIP it over so that the open end is on the bottom of the cage. (keeps them from peeing/pooing on cardboard)
There's also another page that talks about diet and nutrition here:
CavyMadness: guinea pigs and the people who love them