Applying for a 2nd MS degree - Should I mention my previous MS in the statement of purpose?
There are several factors you should consider: 

1) check if you are obliged to inform the university about all your previous degrees (this mainly depends on the policy of the university). For example, I know that in some European countries it is highly important to declare it due to financial issues, where students with similar degrees cannot get funding support. In some African and Asian countries, it is not allowed at all to join a program with similar degree due to insufficient pedagogical seats. For this be careful not making a false declaration. 

2) in case you are not obliged to declare such an information (Note that it does not mean lying about it), it is better to check if you can make it as a positive point. This should be mentioned in your motivation letter, by intuitively indicating your motivation for the applied program and how its combination with your previous MS will help you to solve difficult problems in the labour market or doing PhD. 

If you cannot find such a connection between the two programs and you find that the motivation is not sufficiently convincing, you can hide your degree as long as you are not obliged or asked to declare it. 

Overall, everything will be based on how you illustrate your motivation.