How to calculate marginal utility with two goods?
This is quite simple to answer given you know a bit of multivariable differential calculus. You're looking for partial derivatives of the utility function. 

So, given $$U(x,y) = x(y+1)$$ we have $$\frac{\partial U}{\partial x} = y+1$$ and $$\frac{\partial U}{\partial y} = x$$. 

These are the goods' marginal utilities.