The UR44 has 6 inputs and 2 mix busses (so 2 discrete stereo outputs available)
There are 2 Hi-Z and 2 Line 1/4" jacks on the front (or 4 mic inputs, they are combo jacks) and 2 1/2" jack line inputs on the back.
-> so for you plug one keyboard in front inputs 3 and 4, and the other in the back inputs 5 and 6. You can use 1 and 2 for guitars or microphones
Your two monitors go in the back ports, Main L/R and Outputs 1/2 are the SAME outputs and connect with Mix bus 1 (the reality that these are the SAME is often a little unexpected). Either of these can be the first monitor pair.
Outputs 3/4 go to the second monitor pair.
You can use Cubase to switch monitor pairs in the Control Room if you configure it correctly.
Note the 2 headphone ports differ slightly in features:
headphone 1 connects to Mix bus 1
Headphone 2 can connect to either Mix bus 1 or Mix bus 2.
So if you want to monitor with only headphones and you are using both mix busses for external monitors, you will need to turn your monitors off (or volume down).
Hopefully that is clear (and I didn’t make any mistakes).