I was involved in an interesting Exadata installation last week. We have a client that wanted a half rack Exadata spit into 2 quarter racks to separate development and production. The best part was that we were doing this from the factory image, without the use of Oracle's ACS. Normally, a standard Exadata installation can be a tricky proposition, but since we've run through the process a few times for other customers, it wasn't too difficult.
We started by running the dbm_configurator spreadsheet and giving it the configuration information for a quarter rack system. We then went through the process again for another quarter rack, with an additional storage cell (a half rack can't be split evenly since there are 7 cells). We decided to give the extra cell to the production cluster since it would be running +DATA in high redundancy. We did have to massage the hostnames and IPs a little bit, since the clusters would be sharing the switches, and the IPs would be intertwined between the 2 clusters.
After this, we ran through the typical installation. The only difference was that we had to run everything twice. Once from db01 (production), and once from db03 (would become db01 in development). What we ended up with were 2 separate clusters that only shared the Infiniband network. We had a fully functional Exadata environment to test patching and software code releases.
If your company is looking at purchasing 2 quarter rack Exadata systems that would go into the same datacenter, I would definitely recommend looking at a split half rack solution. Not only do you get an extra storage server and Infiniband switch (half and full racks include a spine switch, while the quarter rack only has 2 leaf switches), you can get significant savings on floor space and power. The power requirements for single phase power are the same between and quarter and half rack - 4 L6-30 plugs. If you purchased 2 quarter racks, you would need 8 L6-30 plugs in total. Also, since (according to Oracle) you are not allowed to place anything else in the rack, you end up with 2 cabinets that only have a few components in them.
Overall, it was a fun experience to go through the installation from the factory image to client handover. We even had enough time at the end to get a DBFS up and running for the client.