well i've thought about the same thing moptop, but for me personally, i think in a secure D/s or M/s relationship adding a sister sub can enhance things.

granted you don't want to involved another person right away but after awhile, after the Dom and first sub have gotten to know each other very well, have lived together, have experience together, i personally think adding another submissive can be a very good thing for both.

i know i personally have always wanted a sister sub. i think if a Dom and His first submissive find the right sister sub, it can help alleviate some pressure from both sides. lol like i use to tease Azrael about how if i had a sister sub we could take our brattiness out on each other before He got home so He wouldn't have to deal with it.

but again it just boils down to what is right for you.......