S7 multiprocessor ?

Hi All;

I have an S7 rack with a CR2 backplane.
This CR2 backplane is split in two halfs, each has one 414 processor.
Then, I already have 2 processors in the rack.

Now, I would like to add a third one !

As soon as it is impossible to have 2 processors in the same " sub rack" is there any way out ?

Rack extension ?
Another backplane ?
... ?

Thanks