@ben8000,
ben8000 wrote:is it legally possible to have the custodial parent open a case in another state while i was already paying child support from an open case in california for the same kids?
It's possible, but the question is whether it is proper. (I'm also a little unclear as to whether this "new" case in the other state is seeking a modification of the California child support order, or whether she is styling it as if it were a brand new order.)
There is a uniform set of laws called the Uniform Interstate Family Support Act (UIFSA), which has been adopted in a number of states (22) including California. I don't know what this other state is, or whether they have adopted UIFSA.
Contrary to what PUNKEY said, the proper jurisdiction for child support is NOT (necessarily) the state where the child is residing, although it MIGHT be the state where the child is residing. For example, if you never set foot in California (let's say you live in Arizona, and the child and Mother moved to California). In that case, California would NOT have jurisdiction over you for child support purposes, unless you consented to it. Proper jurisdiction is where you (the payor of child support) live -- in my example: Arizona.
But, once a state has assumed jurisdiction over the issue of child support (in your case, that appears to be California), then the UIFSA has a law concerning modification of existing support orders from other states. This is a
fairly confusing area of the law, however. If Mother and child have moved from California, then perhaps it no longer has jurisdiction to modify its support order. If YOU live in California, my guess is the order should be modified ONLY in California.
Suffice it to say that a child support order of a state
can be modified by the Court of another state,
under certain circumstances. As Jespah said, you should seek the advice of an attorney who practices in this other state to render an opinion about this.