help my guinea pig is fighting my other constantly

Question
help!!! so i got a guinea pig at the pet co she is so sweet but she would not eat or drink! so we brought her back to pet co for a vet check up she was sick she would not be able to go home for a month!! so we got another guinea pig (one month later) we picked her up and brought her home i tried to get them to like each-other buy letting them play together but when i try to put them in there cage but the other guinea pig will fight her i have to pin the other guinea pig down just to get her off of her now my old guinea pig has to live in a tiny carrying case because i cant get a new cage please please!!!!! help!!!
(one more thing the new guinea pig is pregnant and they live out side in a heated cage)

Answer
okay, first things first you need to try some bonding techniques,

try driving them around in a car in a box together, this scares them together in a sense.
you can give them a bath using guinea pig freidnly shampoo, this removes their own smells and allows them to get used to the other new pigs smell, plus it doubles up as a scary experience to make them bond,when you are doing this you should also clean out the cage with some watered down disinfectant.

try these, for a week or so and each time afterwards try placeing them together in a neutral place, like say a run in the garden that neither has been in before.
after a week if they seem to be getting on okay then try them in the cage together.

if this does not work them please get back to me and ill see what else i can think of

all the best