Clustering between 2 nodes with dissimilar operating systems is not available. A couple of the issues are 1) hardware - how are you going to connect the shared disk. This is possible if you're using SAN technology, and it is possible with scsi, but difficult.
2) software - what application are you planning on making highly available? If its something like oracle, the application needs to be able to read the same data formats, use the same libraries, etc.
Hope this helps abit