merge -s ours question

Here's the scenario,

I have a muddled development branch that has a mix of local development and
upstream merges:

 upstream 1--2--3--4--5
           \     \     \
      dev   o--A--x--B--x ... Y

It took a lot of work, but I backported everything newer than A using rebase
and cherry-pick, and omitting the merges, giving me a dev' clean of merges
from upstream:

     dev'   o--A'--B' ... Y'
           /
 upstream 1--2--3--4--5
           \     \     \
      dev   o--A--x--B--x ... Y

Now it turns out that going forward, I need two development branches, one
that's based on a newer version of upstream for which the merge work was
already done in the original dev. In an ideal world I'd have this history:

upstream 1--2--3--4--5
          \           \
    dev2   \           o--o--o--o
            \
    dev1     o--o--o--o--o

And I would periodically merge dev1 into dev2 (dev2 requires the work being
done on dev1). The problem is that my backported dev' has a mix of dev1 and
dev2 and it's impossible for me to tease them apart at this point. So my plan
was to do this:


     dev'   o--A'--B' ... Y'
           /               \
 upstream 1--2--3--4--5     \
           \     \     \     \
      dev   o--A--x--B--x ... Z

Where Z is created using using "git merge -s ours dev'". Is it correct that
this should do nothing more than give me a new merge base for dev and dev'?
Then going forward I can merge from dev' to dev per usual.

Thanks,

j.