You're have no choice but to use mono. Monitors do not accept stereo inputs.
Each monitor will probably have two inputs. The input will either be XLR or TRS (1/4"). These are pretty much the same input except that they are using a different connection.
The connection is not stereo.
Your monitors will not accept stereo connections.
The connections on the monitor are mono.
You can use balanced, but it will be mono.
You can use unbalanced, but it will be Mono.
You have no choice but to use mono signals. Your signall will end up mono. 1 monitor=1 signal source.
You can spend your time stressing about what you want to use, if mono or stereo, but in the end it will not matter because you have no choice but to use mono.
1 soundcard output will go to 1 of the connections of your monitor. You will not take 2 outputs and plug them both to 1 monitor. You will not take 1 output and plug it into both inputs of the monitor. You will not take 1 output and split the signal to each monitor.