coupled inductor in series
If both coils share the same core, then the length of the core is the same whether one coil is used or two (aiding). If opposed, then flux is much, much reduced.

If the current reduces because the inductance has increased then there is an overall net reduction in flux despite the number of turns doubling - this is because inductance is proportional to turns squared hence inductance quadruples (current quarters) and turns only double.

Magneto motive force (mmf) halves when the turns double because current quarters. MMF drives flux hence there is an improvement.