It appears there is some bug preventing notifications from going through. Because of that, I'm going to go ahead and give you some information. There are many causes of hair loss and skin problems in guinea pigs. Most of them require diagnosis and treatment by a vet. I'll tell you a bit about them in a moment.There is another possibility in your guinea pig's' case.
When guinea pigs share a cage, they usually spend time grooming each other. Sometimes a guinea pig will get carried away with that grooming, and chew hair off. This is called "barbering." Many times, the hair will just be shorter in the barbered area, but sometimes they will remove all the hair to the skin. Barbering occurs most often on the sides, but can occur in any area.. If you want to find out if that is what's going on, you can put a grid of some sort in your guinea pigs' cage to separate them. Or, you can put them in two separate cages next to each other. Keep them close together (but not so they can reach through to each other's fur) because it would be traumatic for them to be totally away from each other. If barbering is the cause of the hair loss, you should see some color appearing on the bare skin within a week, and some new downy fuzz within two weeks. Barbering is not usually a health concern, so you may choose to put your guinea pigs back together even if barbering is occurring.
If the hair doesn't start growing back, there is some other skin problem. Mites (which can be nearly impossible to see) are one possibility. Bacterial or fungal infections are other frequent culprits that cause such symptoms. Hormonal imbalances are less common, but can lead to the same thing. Very old females are most likely to have hormonal problems. There are even more possibilities. To read about the various possibilities:http://www.guinealynx.com/hairloss.html
All of these parasites and conditions require treatment by a vet. There are remedies you can buy in pet stores, but they don't work, and if you pick the wrong one, it can make things worse. This link will take you to a directory of vets who are familiar with rabbits and guinea pigs:
As for the weight loss, hormonal imbalances can lead to weight loss. Mites can drive guinea pigs so crazy that they fret and scratch until they lose weight. The most common cause of weight loss is a dental problem. Because your guinea pigs are suffering from both fur loss and weight loss, it would be best to schedule a vet appointment. However, if you want to try separating them first, that is another option.
If you have more questions about this, just let me know by clicking on REPLY. I hope your guinea pigs will be fine.