2022 Update
At present, it looks like one of the best solutions will be the Cielo Breez Eco. This is a thermostat which can substitute for an air conditioners existing IR remote. There is a manufacturer – provided cloud to cloud integration available in the smartthings app. Just go to “add a device“ and search by brand for Cielo.
https://www.amazon.com/dp/B08RB2Q88Y/
Also available in other countries.
Older information, some may be out of date.
You probably won’t be able to control both with one harmony hub unless it’s a smart TV and isn’t using an IR remote or unless you use an IR extender. The IR extenders that work best are the ones that run via cable, and there are people that put them in several rooms that way, but then it means you have cables running along the walls (or you have to go to the trouble of running them through the walls) and many people prefer just to buy an additional harmony hub for the other room. This typically costs around $99 for just the hub at either Amazon or Best Buy, but it does go on sale occasionally for about $89. So definitely shop around.
https://www.amazon.com/dp/B00N3RFC4Q/ref=asc_df_B00N3RFC4Q4998800/
There are some IR Bridge devices which communicate wirelessly between two of the bridges, but I haven’t found them to be very reliable and again, it’s usually just easier to buy a second harmony hub.
The reason for all of this, as I’m sure you already know but someone else reading this might not, is the IR has to be a line of sight to the device that it’s controlling. It doesn’t go through walls the way Z wave and WiFi do.
I like the harmony hub and use it at my house, but it’s not the most intuitive device when you use it with an air conditioner. You might instead consider the remotec ZXT-120. This is a device that will look like a Z wave thermostat to SmartThings, but has its own IR and is designed specifically for air conditioner control. It still has to be line of sight to the air-conditioner, but the interface is just a little more intuitive. There are a number of community members using it so you can ask more in one of those threads if you’re interested.
There’s also a new inexpensive WiFi to IR controller, the zmote, that some community members are using, but it’s going to have the same issues as the harmony: it’s not a very intuitive interface for an air conditioner.
I would say if you already intend to have a harmony hub for the room the air-conditioner’s in, it’s worth trying that first, but if you’d rather have a more thermostat-like interface, you can consider the remotec. ( I haven’t heard of anyone specifically using the zmote for an air conditioner yet, but it’s another possibility.)
And if you’re having to buy a device just to control the air-conditioner, then maybe consider the remotec first.
Other options: smart air conditioners
I’ll also just add this for people who might be reading this thread later based on the topic title that there are some smart air conditioners and if you’re starting from scratch, I think I would look at one of those first as the most elegant solution. (The GE Devices available in the US have an IFTTT channel, And there are some Samsung devices but they are not available in the US yet.)
But if you already have an air conditioner that has an IR remote, either the harmony home hub or the remotec ZXT120 can give you good integration with SmartThings.
There are a couple of other retrofit wifi solutions, like the tado, and I had high hopes for those, but so far all of the community reviews have been bad on those. Hopefully if there are people using any of those successfully, they will also chime in. 