Inductors that are completely isolated, add in series and reduce in parallel just as resistors and capacitors do (well, capacitors do it reciprocally because their impedance goes as 1/C, but, you know).
Inductors that are coupled, multiply in series (aiding or opposing), as the formula gives. In parallel, if they are different numbers of turns, then you are creating a short circuit through the difference of turns; if equal turns (and same direction), nothing happens at all.
Parallel, well-coupled windings are easily understood: suppose you have a helix of wire on a core. Suppose you split the wire lengthwise into two pieces, in place. Are they not the same wire, whether measured individually or connected in parallel (restoring the original single wire)?
Tim