You have to install Ubuntu using the Alternate CD.
When you come to the partitioning screen, select manual. Then format both your 750 gig HDs selecting "Volume for raid" as type (or something like that).
Once done, in the partitioning screen you will see another entry called "Configure software RAID", where you can choose RAID type and drives used.
Then you will see a new "partition", /dev/md0, which is your raid array. Format it as you wish, you can even use LVM on that.