It is never a good idea to "backup a backup"... because any errors that occur in the original backup just propagate over to the new media. So from that standpoint, it does not offer any significant new protection.
From a practical standpoint... small changes to backup data can cause larger changes in the backup set. Hence... if you are backing up that "backup"... then you can often end up with a multiplying affect of how much data is being backed up... especially to the secondary backup. In a case like backing up to Crashplan+... that is valuable network bandwidth that you are consuming.
Finally (and you are doing this correctly)... you are MUCH better off using a totally separate and independent backup to the second destination... ideally, using a different backup program. All backup programs have their pros & cons. All of them can suffer from programatic or human error. By having a set of different and independent backups... to different destinations... is the ideal situation.
I believe that you are using Time Machine & Crashplan+. This is perfect. You have two different backup programs, backing up to two different destinations, one of which is offsite, and both of them are "hands off" meaning that if (or better said when) you get lazy and forget to do something... your data is being backed up.. A great solution.
The only thing I do extra is make a clone copy of my primary media. I use CCC for that. I generally do not like clones (because they do not provide effective versioning)... but since TM & CP+ does versioning for me... I am covered. I back up my media (Aperture Libraries, FCPX projects, Personal video media, etc) because that it irreplaceable data, and in the event that I have a major computer failure, I can simply move this media to a different computer in my house and keep working.
/Jim