Secure MultiParty Computation with secret inputs for secret outputs I want to know if it’s possible to use a SMPC (Secure Multi-Party Computation) to have $N$ entities compute the outcome of a known mathematical operation with two or more secret inputs, where each secret input is only known by one entity in the $N$-sized group.

I’m asking, because I want to define a protocol where two network nodes take part in a virtual money transfer – with $k$ witness nodes – where the balance of each node taking part in the transaction is kept secret to itself. After the transaction is calculated, each part can verify with the outputs whether the operation was successful or not.
Yes, that is possible -- that's exactly the problem that secure multiparty computation solves.  You should start by reading standard references on secure multiparty computation.

You might enjoy the following paper, and follow-on work:

Secure Multiparty Computations on Bitcoin,  Marcin Andrychowicz and Stefan Dziembowski and Daniel Malinowski and Łukasz Mazurek, IEEE Security &amp; Privacy 2014.