I usually take into account the time between series, whether they have different composers, etc. It gets kind of complicated. Since R2 is a direct sequel with only a one year gap in between with almost all the same information in terms of its music, it's fine with me that it has one member list. I'm not as familiar with Saiyuki, but I'm thinking it's one of those things that would have separate lists, if only because if I remember correctly there is at least one other Saiyuki series? Correct me if I'm wrong, haha. XD There's always the option of coding the join form to give them the choice to join one or both, too, if you haven't already done that.
If you do have reason to believe they should be one list, let me know why, though! Like I said, I don't know much about Saiyuki. XD