Using TPL.DataFlow blocks, is it possible to link two or more sources to a single ITargetBlock(e.g. ActionBlock) and prioritize the sources?

e.g.

BufferBlock b1 = new ...
BufferBlock b2 = new ...
ActionBlock a = new ...

//somehow force messages in b1 to be processed before any message of b2, always
b1.LinkTo (a);
b2.LinkTo (a);

As long as there are messages in b1, I want those to be fed to "a" and once b1 is empty, b2 messages are beeing pushed into "a"

Ideas?