You need a third element in this equation - you can think of a FaultSet as of a 'big SDS' - in order to form a cluster, you need 3 SDS, or, in this case, 3 FS. You can mix them, so for example you can have 2 FS and a single SDS, 1FS and 2 SDS, 3 FS etc., but in general FS acts as a single cluster member, so you need at least three.
In this case, I would suggest you to move one machine to a third rack or add another 1-2 nodes and create third FaultSet.
Please check out ScaleIO User Guide, under 'Fault Sets' sections there's much more details about the FS.
Hope that helps!