Thanks for the replies. Yes, I realize that "separate interest" is a bit of a legal fiction or a metaphor used for characterizing an assignment that behaves in a certain way, but it is not a creation of the law itself. Honestly I wish that the statutes relating to QDROs had a bit more specificity about what is allowed, or that there were regulations with better examples.
I am having some confusion or whether a "separate interest" benefit survives if the participant dies before either party commences benefits. I thought that one of the favorable features of a separate interest vs. a shared payment is that it would persist afterward irrespective of the participant. Is that not the case?
I know it's a slightly different context but I'm reading the PBGC's manual on how it administers QDROs, and under Section 10, "Surviving Spouse Rights of Alternate Payee," it states:
"When PBGC administers a separate interest order, it uses a totally severed approach. The participant's benefit is divided into two separate parts - one for the participant and one fo rthe alternate payee. Once the order is qualified, the participant's death before or after the alternate payee's benefits commence will not affect the alternate payee's rights to a lifetime benefits. Thus, assignment of either pre- or post- survivor annuities are not needed to ensure that the alternate payee will receive lifetime benefits."
"However some alternate payees and participants may choose to include survivor annuity provisions in their orders." It goes on to basically address my original question and say that "Survivor benefits are in addition to a separate interest or shared payment the alternate payee also has a right to receive."
As to calculating the amount of the QPSA, this agrees with what you're saying, QDROphile, that the order can assign any amount of a QPSA. Their model order leaves a blank percentage in the surviving spouse provision, to allow any percentage of the QPSA to be assigned- then, to in a separate interest, the QPSA is (assigned percentage) x (participant's retained separate interest) x (survivor annuity percentage provided for in the Plan). So, with the facts I gave above, the QPSA would be 50% of 50% of 100%, or 25%; but, she will also have her 50% separate interest as well.
I think that's the best answer I'm going to get on this.