I don't know if you read it, but I put together a guide on how to breed for Deviants here: http://beastkeeper.com/thread/5218/1#43165
You need to get a baby with two of the same pattern genes if you want a pattern deviant. So yes, if both parents already have the same pattern gene that definitely helps. Ideally both parents will already have two of the same pattern genes, so your baby is guaranteed to get the required genes (let's say you want to breed for Zebra deviant. Both parents look like Zebras, but the mother has Cream as recessive gene. This means there is a 50% chance that the mother will pass down her cream gene, which would mean that the baby doesn't get double pattern and won't be a deviant). Now if you do manage to get a baby with double pattern genes, there is a 10% chance for it to be a deviant. So even with the perfect parents it can take quite a few breedings before you get what you want.
As for the last question, deviation doesn't get inherited. Therefore, if you breed two pets with the same deviant pattern, your baby will again have a 10% chance to be deviant, just like with normal parents. However, if you breed a Unicorn with a deviant pattern and a Uni with a deviant colour, you have a (very slim) chance of getting a double deviant.
As for the last part: Short answer: Yes, these are perfect parents to try for Deviations. There is a 10% chance of getting a deviant pattern out of those two parents (and a 10% chance for deviant colour but you mentioned you're aiming for pattern deviation).
Long answer: Both of them are silver gray, which is the least powerful colour / pattern combo. Even without having the genes researched you can be 100% sure that both of them will have double Gray and double Cream genes (if you check the Skin library, silver gray is at the very bottom right, which means any other gene will always be more powerful and "overwrite" it). This means that any baby from this pairing will ALWAYS have double pattern and double colour genes, giving you a 10% chance that you get a pattern deviant and a 10% chance that you get a colour deviant Uni.
