Any drawbacks or things I should know or worry about?
Yes, you are gaining an illusory additional amount of protection by cloning the TM drive, maybe just an extra 1% rather than the 100% you probably think you are getting. You should use an entirely different backup method, such as CCC, to backup to the second drive.
Reason being, if there is a TM software issue affecting restore, it wont help you having an identical copy of the TM disk as you'll have the identical problem
This actually happened to me a few weeks back which I posted about here, I wanted to recover a file which I had accidentally overwritten, and for whatever reason, it wasn't on my TM backup, though it certainly should have been. I was able to go to an online backup service I use and get it from there. Had I merely had a duplicate of my TM drive, (or even 100 of them), that would have done me no good at all.
I use TM to an external HD, weekly backups to an online service, and monthly clones (CCC) from the internal to a second HD,