If you really only have two physical drives, they cannot be constructed into a RAID 1+0 set at the controller level. You need a minimum of 4 drives to create a RAID 1+0 set at the physical controller level.
With two drives, you could create a RAID 1 set, which will leave you approximately 160GB of usable storage.
This is great to help provide fault tolerance at the disk level for your server (i.e., your server will be able to survive the failure of one of the disks). However if they are both on the same controller, a controller failure will result in downtime on our server.
There are pros and cons for partitioning this into two parts as you describe. It all depends on how you want to manage your servers. It is certainly much simpler for most people to just manage (setup, backup, restore, etc.) a single partition. And splitting it will not provide you any significant benefits for performance or reliability.
On the other hand, your administration practices might make more sense to separate the RAID set into multiple partitions, with the larger one for user files. Sometimes for recovery, it is desirable to get the server up asap, and worry about the user date later. In that case, a separate partition for the OS will allow a faster recovery of the server to a running state.
I assume that you know you'll still need to perform regular backups.
The article "Introduction to RAID" will help you if you have any questions specific to RAID.