Where to pour bleach solution to unclog AC drain
I wonder why you have two drains. My evaporator has two caps on drains but a drain only fitted to the lower one. I thought maybe the 2nd one was for installing the evaporator in the horizontal, rather than the vertical.

Is your drain backing up? If not, maybe you don't need to get bleach into the condensate drain. My installation (done in 1991) does not have a Tee to get bleach into the condensate drain and we have never had a backup of condensate. I do wish I had one though. The same company that installed ours did put a Tee into the condensate line in another house we owned at the time. A stopper is inserted to keep air from flowing out at that point. It is nice to be able to remove the stopper to see if the condensate is flowing. Bleach could poured into the line there but we never did, nor has the new owner of the house.  

You could drill 1/2" holes in the horizontal runs and insert stoppers to prevent air escape during operation. With a small funnel you could introduce bleach into the system with the fan off. Most of it would flow downstream but enough of it might flow into the evaporator to kill mold there.