You are correct, if the 2 subs have the same impedance then 2 in parallel will be half, 2 in series would be double.
Series always adds, regardless of the numbers, example 4 ohms + 2 ohms = 6 ohms. Works if the 2 are the same or not.
Parallel is different. It's the "reciprocal formula"... total = 1/ (1/R1 + 1/R2).
So a 2 ohm and 4 ohm in parallel is 1/ ( 1/2 + 1/4) = 1.33 ohms.
For 2 coils the same it comes out to 1/2 each but for anything else you need a calculator. Easiest thing to remember is that series adds, and parallel will always be lower than either one.
Hope I didn't confuse you more.
1999 Buick Park Ave Ultra
Head Unit: JVC KWHDR720
EQ: Audiocontrol EQL
Front Stage: Hybrid Audio Technologies Imagine 5.25 mids, Morel Maximo tweets
Sub Stage: Boston G3 10", 0.5 cu ft sealed
Amp: PPI Phantom 900.4
Rear Fill: Boston SE953 6X9's