The dual-processor models incorporated a pair of quad-core processors for 8 total cores, or a pair of 6-core processors for 12 total cores.
The problem is that the processors that can be used for easy upgrades are no longer being sold new.
Adding two (single-processor models) or four (dual-processor models) processors makes a lot of sense, and definitely provides the best bang for your buck.
Remember that all of the processors you can use for upgrading the 2010–2012 Mac Pros make use of hyper-threading so that a two-core upgrade will be able to run four processing threads, not just two.
Bumping up from a quad-core to a 6-core can be an effective way to get a few more years out of your Mac before you need to consider replacing it.
As requested by readers, this single page provides direct links to upgrade related Q&As for G3 and later Macs for your convenience.