I'd like to buy 2 USB hard drives and simulate a RAID0 array i.e. plug both drives into my MacBook Pro and have Time Machine back up my data across BOTH drives so that they're mirrors of one another. Can you do this with Time Machine ? I know I could buy a 'proper' RAID back up solution but just buying 2 drives is cheaper...
I'm not sure to understand what you need.
RAID-0 consists in grouping several hard drives in such a way that a single volume appears on Mac desktop with a size being the sum of all the sizes of each unit. You can do that with Disk Utility, but your system is less reliable than a single bigger disk (a failure from any of your disks could lead to loss of all data).
Mirroring is obtained with RAID-1, which is possible which Disk Utility again. The size of the RAID volume is the size of the smaller unit.
Time Machine does not mirror hard drives. It keeps an history of changes in the filesystem.
If you want a Time Machine volume that is mirrored on a backup volume, you can use SuperDuper! backup software.
If you have two disks that you can use for backups, it might be a very good idea to use two different backup utilities, too. For example, use disk 1 for Time Machine backups, and then (independently of that) set up Carbon Copy Cloner to take backups to disk 2.
Time Machine is not perfect, it has its bugs, and in some situations it may fail to back up everything properly. Simply mirroring or cloning your Time Machine backups doesn't provide any protection against that kind of data loss; moreover, if your backup volume gets corrupted, cloning it will clone the corruption, too. However, if you use two very different kinds of backup tools (say, TM and CCC), it's much more likely that you have at least one full backup available.