How can a Mac split a single backup to two independent, external drives
Time Machine can't partially backup to one drive, then when space runs out, continue to the next drive.  

But you can concatenate several drives to look as one and then use Time Machine. 
As you have said, you can create a RAID drive. This should work with Time Machine. Recovery would look normal. (See here). You can restore from Recovery, Migration Assistant, etc.)  Time Machine would detect it as a RAID drive and will mount it as one drive instead of two.

When restoring the entire of the drive I would always suggest booting to Recovery instead of migration assistant.

Just remember, using RAID always significantly increases chances of corruption or failure. RAID 0 is the worst because there are no failsafes: If one drive fails, corrupts, or dies they are both useless. 

Your best bet would to purchase a single big enough drive to do the job. 

Remember it always better to do it right, especially when it comes to peoples data.



If you are looking to purchase a drive, I would go with LaCie, and definitely not Seagate. 4/5 Seagate drives have failed without being moved within 6 months.  None of my LaCie drives have failed.

Note: I am not affiliated with Seagate or LaCie and will not gain in anyway (financially or otherwise) from the purchasing of their products.  Additionally, the suggestions above are form personal experience and do not in any way reflect proven published statistics.