Anyone have any luck installing a second AC?

Wanted to ask if anyone has had moved from a one split AC unit serving two floors (2800 Sqft) to installing two Air conditioners, one for each floor?

Our AC despite being brand new does a shitty job cooling the second floor (can’t get the second floor to go below 75 if it’s 80+ outside).

So now we’re thinking of buying a second AC specifically for the second floor and im wondering how much work this would entail?

Or should I investigate why my current AC is not cooling the second floor as well?