In any case, the formulas are the same.
Assume we want to parallel two sets of three 4 ohm speakers in series. Three 4 ohm speakers in series gives us 12 ohms. The impedance when two sets are paralleled is:
1/(1/12 + 1/12) = 6
That happens to be 1/2 the impedance of one set in this case (divide by two as OP noted), but that is not always the case. In fact, you can count on it not being the case most of the time, unless you are paralleling two sets with the same impedance.