As a Mercy, it always feels like if the other healer is a Zenyatta, I'm the only one actually doing much healing...It's annoying :/
Somehow gettings Comp Wins..
A Mercy that's on the ball with healing is usually the ONLY person I keep the harmony orb on unless someone is in dire needs.
Zen doesn't do much healing
He does 50% of Mercy's healing from a greater distance and for 3 seconds around corners while also debuffing and dealing a ton of damage. Yeah his healing is half of Mercy's, but he's still a pretty damned good healer considering how many other things he brings to the table. At least his ult synergizes with doing his job instead of being a counter for when you fail at your job.
When I play Zen with a Mercy, I usually put my Harmony Orb on Mercy. She's my default target when no one else is about to round a corner or at Critical HP. She should be the one doing the healing, but she should also be alive as much as possible to be able to do it.
Also a lot of people just suck at healing with Zen in general, so it might just be that.
Since he has to multitask on 3 things, sometimes people get tunnel vision for just debuffing and killing and forgetting to take care of keeping that 30hps a sec on someone who needs it. Another reason why I just like to stick it on Mercy so she can survive to heal. Keeps me focused on one task.
People need to know their role. Single target healing power goes Lucio<Zen<Mercy<Ana. If it's just me as Zen or me and a Lucio, I know that I need to be paying more attention to healing as primary task and just keeping a discord orb out there to help and attacking when I have time. If there's a Mercy or Ana, then I know I can mostly harmony orb them or in the case of Ana, someone difficult to hit like Genji or Pharah and focus on killing/debuffing.
There's also little tricks to get over the fact that sometimes people are in your way. Since you can't harmony the same target twice, a second press of the heal orb button will allow you to hit a second person further away. Sometimes if I see a long range person hurt and someone is in the way in front of him, I'll automatically just press the button twice and it will definitely go to the further hurt person.