What material to tie two cinder block walls together?
The normal way would be reinforcing bar around insides of the blocks, then add concrete. This is called a &quot;bond beam&quot;. Blocks with open ends are used.  If you use enough reinforcing it can be very strong.
For your way stainless threaded rod, you probably need to go to a fastener specialist for that. (or stainless rod and you thread the ends, but threading stainless is hard)
or stainless, or aluminium flat bar and you drill the ends and put bolts through crosswise. aluminium will probably last 5-10 years, stainless much longer.