I'd do a bit more digging on the background of the pair - chances are that the Dutch breeder is a member of this forum. For me the crucial information is whether you have a sibling pair or if they are from different parents - there's already enough inbreeding in most tetrataenia lines.

I'd try to get them eating some fish as well as the rodents. I use whole frozen smelt and my little guy loves it. More variety in the diet is never a bad thing. How heavy are they? In the photo they don't look skinny so they are obviously feeding well.