Wire in series and you add total resistances. equation= Y=R1+R2 you can do this with any amount of subs or voice coils. So two .5 ohm voice coils would be 1 ohm in series. Two DVC .5 subs would be 2 ohm.
Wired in parallel and you use the equation 1/((1/r1) + (1/r2) ) again you can use this for as many subs or VC as you like. 1 divided by .5 is 2. you do this twice and then add together which gives you 4. then your equation is 1 divided by 4. which equals .25
so in short
wired in series=1 ohm
wired in parallel. 25
for future reference my friend. always good to know these things.