I have a strategy which works for me.
I keep a separate "Archive" drive to which I move files which I no longer need on my startup drive and/or which are for "reference only," such as completed project files, manuals, installers, and the like. This Archive drive is regularly cloned (automatically) as a backup, but excluded from Time Machine.
Using this method, I can still get more than a year's worth of TM backups on my 500GB Time Capsule, which is only twice the capacity of the backed-up data from my startup drive, and TM Is available to restore whatever I may need.
Some variation of such a strategy might work for others.
There is another circumstance in which it might be helpful for some users to think about a "special" strategy:
Remember that Time Machine stores every new version of a changed file, every hour. So if someone is working with large files such as videos or large publications, intermediate versions of such files can use up a lot of space in backups.
My strategy for working with large files is to keep the original (of course) in a directory that is backed up, and also, naturally, the "final" version, but to keep interim versions in a folder which is excluded from TM.
You were right, it didnt work....
The second drive could either replace the first or work as an alternate pair with the first.
My next plan is to get a new drive which is larger enough to take all my files and try setting it up as the alternate pair thern leave one connected and only connect one weekly as I'm hoping this might give me the best of both worlds.
I want to use Time Machine to a drive connected to my iMac to give me near real-time protection then take a weekly copy to a drive that I lock away in my safe.
I will keep you posted.
FWIW, I use a two drive TM backup strategy with my iMac: one is always connected & the other I just connect about once a week; otherwise it lives at a different location. This works well -- whenever I connect the second drive, it is used for the next backup.
If I forget to connect the second drive for 10 days, I get a reminder popup about that, even though the always connected backup continues to make hourly backups.