I have read alot about raising littermates together. My parents and I purchased 2 female littermates. We do not raise the girls together. However, we do let them play with each other about once a week. Could this cause a problem?
The big issue with raising littermates is that they are around each other all of the time. If your two pups are only visiting for a short amount of time once a week you should have no issues having to do with littermate syndrome.
I wouldnt have thought so, the whole issue as has just been said with littermates is their continued proximity, a once a week visit isn't going to engender the same rivalry, but rather it will be just another playdate. Saying that, just because they are both female, both Dobes and just happen to be sisters doesnt mean they will necessarily get on. So keep an eye on them when they meet up.
A whole lot depends on how dog agressive the individual dogs are or the bloodline in general, as I find dog agression to be pretty genetic. I usually would not worry too much about two females, but I would never expect two males to get along as adolescents or adults whether they are related or not.