It had nothing to do with the guy behind the character, as he's a good player. It also didn't really have anything to do with me not being used to healing with him, as I've teamed up with other Sages and Scoundrel healers with no problems. But running 8-man progression content with two Combat Medics really threw me off my game and caused me to derp around a lot more than usual.
Mind you, going into an operation with two Commando healers has never been ideal, but I remember doing progression with that setup back in EC NiM and having a good time. Ironically it seems to me that the Trauma Probe changes that made us stronger healers overall are also what makes it more annoying to be teamed up with another Combat Medic these days than it used to be.
Even when I only have to keep track of my own Trauma Probes, it's quite challenging due to the UI not being at all helpful in that regard. The operations frames can't show more than four buffs at a time, while anyone in a progression ops will have at least six buffs on them at all times (the four class buffs, sprint and a stim). So you can't expect the ops frames to reliably show you any information about your Trauma Probes, ever. (Considering that they make up about twenty to forty percent of my healing these days, I'd say that being unable to track that particular ability makes for quite a handicap.)
The reason I can still manage on my own is that once you know the fight, you can develop a feel for when Trauma Probes need refreshing, who uses theirs up quickly and whose only needs refreshing after it expires from lack of use. When you have another Combat Medic casting Trauma Probes as well however (which don't stack and therefore overwrite yours), the whole thing just turns into a confusing, unpredictable mess. As if to mock and confuse you just that little bit more, your character will also wave her hands around in a casting animation every time your Trauma Probe gets overwritten, even if you're in the middle of doing something else.
Healing is not a science of exact rotations, so I figured that it shouldn't be that susceptible to disturbances in one's casting priority as dps can be. But even if healing is more of an art, healing with another Commando just feels wrong... as if we're both trying to paint two different pictures on the same canvas, at the same time, each person constantly painting right over something the other one put down only a minute ago.
I'm surprised that it took me this long to get really annoyed by this, especially since I definitely recall running with other Commando healers since patch 2.6, but that was mostly in casual 16-man ops, where a) healing efficiently wasn't as crucial and b) one person can't keep the whole ops covered in Trauma Probes anyway (or at least not very well).
I don't necessarily expect Bioware to make Trauma Probes stackable, but at the very least they could:
1) fix it so my character doesn't do casting animations when other people refresh my Trauma Probes - it feels a bit like having my character hijacked, and
2) fix the damn buff UI so that you can filter out things like the class buffs and sprint, because in the middle of combat I don't care about those anyway (unless someone died and needs rebuffing after they received a combat res, but that's comparatively easy to keep track of anyway).