I am not 100% sure if this is the right approach, but currently, it is not possible for downstream elements to increase the number of minimum buffers in both decoders when using a v4l2*alphadecodebin element. This is because the allocation query of the alpha decoder is always handled by alphacombine, without forwarding it to the downstream elements. In my opinion, alphacombine should just forward the allocation query of both decoders so that downstream elements can set the same parameters on them.
cc @ndufresne