Come On, Louise, Don't Blow It.

I'm a tad concerned about Louise... And about Thelma, but more about Louise...
Louise (our black dog) is between 5 and 7 years old. When we adopted her and Thelma from Wetnose in 2010, we were told they were about 3 years old, but within a year the hair on Louise's chin was going grey so we figured she was at least a year older than that, if not more.
In 2011, I started noticing some oddities about Louise, but it wasn't until late in the year that she was properly diagnosed with Canine Epilepsy and put onto medication. Without her meds, she suffers grand mal seizures- severe epileptic fits. Its scary to see and you can do nothing during a seizure but make sure they're safe from falling in the pool or some such. As Louise's Epilepsy has progressed she has had a few seizures since starting meds and her dosage has been changed, and it takes her longer and longer to recover from a fit each time she has one.
According to the vet, the seizures do not cause brain damage and she should be okay as long as she's treated.
But in the last few months I have noticed changes in how Thelma and Louise behave around each other.
Let me just stress, they're both healthy (apart from the epilepsy). They both eat well, their poops seem normal, they're not aggressive and they are really well behaved.
They are mostly outside dogs, and they have a doggy igloo on the front stoep that they share, with blankets and a big cushion. They've never been cuddly girls- never snuggling together- even when its cold, so I always made sure that inside or out, they had enough space to be comfy without necessarily touching each other.
The first thing I noticed that was odd was them snapping at each other when they're in the igloo together. Unfortunately I haven't been able to see who is snapping at who because when I go to look they stop.
But it didn't seem serious, so I didn't try and do anything about it.
Then this last week, I noticed Thelma has taken to chilling in the shade at the far end of the back garden. Now this is very odd to me. Thelma is a fabulous guard dog, barking only when there's something worth barking at, and the front stoep has always been her place.
If Louise barks at something, Thelma comes running round the front to see whats up and join in, but when they both go to the gate Louise snarls and snaps at Thelma!
Does anyone have any suggestions or advice?