Is it possible to backup between 2 external drives?

As your data is so important to you I would do the following.

1. Buy a NAS with hot-swappable RAID disks as your primary backup. Will also have backup software.
2. Don't just rely on Time Machine. Use Superduper or similar as well as Time Machine. You can schedule backups for a time you're not working. Backup at least once a day.
3. Also backup every day to one of at least two portable external drives ( 7 would be better, one for every day of the week) and store at least one of these somewhere other than your office/home or where your NAS and Computer live. Rotate the disks so that you have at least one somewhere else and swap them at least once a week.
4. Test that your backups will restore successfully. There is no point backing up unless you know you can restore.

 

Stewart21 offers some excellent advice. The frequency of backups also depends on the volatility of your data. If you don't make that many changes in a day, you may not need daily off-site backups. You need to assess your risk.

I don't think you're paranoid about backups. Multiple backups in multiple locations is the way to go. Be sure to test them to verify that you can recover the data. An untested backup is not a backup.

 

You might consider getting a USB3/SATA docking station, and a number of "bare drives", and build your backup solution that way.

To see what I'm talking about, go to amazon.com and enter "usb3 sata dock" in the text search box. You'll get many choices. They are inexpensive -- $25 or even a bit less.

The advantage of a dock-based hardware setup is that you can swap backup drives easily. If one drive goes bad, just switch to another.
If the dock has a problem, just replace with another dock.

I would suggest you consider using CarbonCopyCloner as well as Time Machine.

TM may be useful as a minute-by-minute, hour-by-hour backup at the computer, but a CCC clone backup is far more useful if you encounter an "I can't boot!" situation.

Keep a working backup close to the computers. TM is good for this.
Keep a bootable clone nearby. CCC is what you want here.
Store an "archival backup" away from the computers.

 

Hard drives are reasonably cheap.

First step would be to have two backup drives for Time Machine. That's a built-in feature of Time Machine nowadays; it will take a bit longer because when you change a file it is backed up to one drive in the first hour, and to the second drive in the second hour, but that shouldn't be a problem.

Now a real danger is that data is corrupted on one drive, and then backing up the corrupted data to ten other drives doesn't actually help in any way. Time Machine helps to some degree because (a) if your Mac didn't notice that the data was corrupted then Time Machine will keep the original uncorrupted data, and (b) if your Mac noticed that the data was changed then Time Machine will back it up, but keep the old data for some time. So buy two backup drives that keep a year worth of data.

Then if you are worried about old data: After a year, replace your two backup drives with two brand new ones and stash the old ones away forever.