Remember that the merles can't be bred together - the puppies are all based on genetics, just like real life, so crossing two merles risks getting a 'double-merle', which is usually pure white and almost always blind/deaf/plagued with health problems. You have the exact same chance of getting merle puppies from crossing a merle with a non-merle, because merle is a dominant trait, so don't worry - there will be plenty of merle puppies.

For anyone who's interested in genetics, I've written out the crossings and their outcomes - each 'letter' represents what type of merle genes that dog has:
MM = double merle
Mm = merle
mm = non-merle
If each litter has four puppies:
mm x mm (like Flash and Jazz) = 4 mm
Mm x mm (like Kip x Jazz) = 2 Mm and 2 mm = 50% chance of merle
Mm x Mm (like Kip x Zeph) = 1 MM, 2 Mm and 1 mm = 50% chance of merle + 25% risk of double merle