Are both units set to the same command, ie both on cool, heat or auto?

There is a setting issue the settings are not coordinated. Check the settings and see.

https://www.minisplitunit.org/

1. Broken Blower

The blower is sort of what it sounds like—a fan that blows air through the unit. It’s a crucial part of the system’s operation, so if the blower motor fails to operate, you can bet that it’ll have an obvious effect on your cooling power.

In addition to reducing cooling power, a broken blower can increase the possibility of a frozen evaporator coil. If the cool air in your unit can’t expel itself, it will bring the temperature so low that the evaporator coil will start freezing over. This will prevent refrigerant from flowing through the AC properly and can lead to more serious problems if left neglected.

2. Refrigerant Leaks

This problem is not unique to ductless systems, but it’s an important one to keep in mind for any refrigerant-based air conditioning system. Refrigerant is the lifeblood of your system. If a leak in the line is causing it to lose refrigerant, that’s going to be big trouble for you.

Until the leak is big enough that you can spot it yourself, there are a few other signals that can notify you of a refrigerant leak:

A frozen evaporator coil

Reduced cooling and heating power

Hissing noises

Tiny bubbles coming from the afflicted area of the refrigerant line

Refrigerant leaks aren’t too common, but it’s far more likely to happen to the indoor units due to formicary corrosion. When volatile organic compounds—such as those found in various household chemicals and products—mixes with the copper refrigerant lines, it can create a reaction that eats through the copper.

3. Condensate Line Clogs

Air conditioners naturally create condensation. Your typical central AC uses a condensate line, a pan, and a drain to get rid of it.

But for a ductless system, consider that the condensate line must run from the air handler, through the wall, and into the outdoors.

If mold and bacteria grow in the condensate line, it can create a clog. Left unchecked, this can cause some water damage to your wall and, in an extreme case, cause the unit to fall off of the wall. Fortunately, you should be able to notice this long before it becomes a disaster as long as you keep an eye on the unit and the condition of the wall around it.