The first IDR-frame as reference for the first 2 B-frames

[Mitra]

H.264 specs says :

8.2.4.2.3 Initialization process for reference picture lists for B slices in frames: When this process is invoked, there shall be at least one reference entry that is currently marked as "used for reference" (i.e., as "used for short-term reference" or "used for long-term reference") and is not marked as "non-existing"

(thanks to nobody555 from the stackoverflow for find it)

As you can see ,only one I/P frame can be used as reference for B-frames. it means the same reference frame can use for both RefPicList0 and RefPicList1, but the document does not have enough details about it.( or it is difficalt to understand at least to me)


my question:
How to set the first IDR-frame as the only reference for the first B-frames in the open-GOP stream with this pattern:BBIBBPBB...? / How to implement it in the x264/libx264 source code?

Note: I want this to be done for the first two B-frames , not the rest of the stream.

(sorry if my English is bad)