Time Machine separate backups across 2 drives
So if I understand correctly, you want to exclude the 1TB working storage volume from the backups on the 1TB backup disk, but not from those on the 1.5TB backup disk. I don’t think this is supported by tmutil’s addexclusion verb, as it does not provide a way to specify that you only want to exclude the item from a specific backup disk.

To hack a solution yourself, you could basically, before making a backup, invoke either addexclusion or removeexclusion depending on which disk you will be backing up to. If you don’t plan on having both backup disks connected at the same time, you could automate this by running a script when you connect one of the two backup disks, see the question on “How to run Applescript on Disk mount” for possible solutions. If you do want to have both backup disks connected at the same time, you will probably also have to turn off automatic backups and hack your own solution to periodically run the startbackup verb (using its -d argument to alternate between the two disks).

Note that I didn’t try hacking this myself, so there might be “caveats” I’m not thinking of. One thing I’m not sure about is whether or not toggling the exclusion on and off again for the 1TB working storage volume will cause Time Machine to do a “deep scan” of the volume again on the next backup, which would make your backups take longer, depending on how many files and directories it holds. In the end, you’ll have to evaluate yourself whether hacking this is worth the additional hassle, given that an external 2TB USB drive can easily be had on Amazon for 65 USD or less.