The nominal 8 inch speaker impedance can be 4 ohms, 8 ohms or 16 ohms. It depends on the make of the loudspeaker not on the 8 inches.

The apparent impedance looking into a transformer from one side will not be the same as looking in it from the other, which is why percent impedance is used.

If you are looking from the high voltage winding (I'm labeling #1) to the low voltage winding (#2), you must scale the percent impedance as follows:

(% impedance) x (Winding #1 nominal voltage)^2 / (transformer base VA)

Your simplest scheme would be to wire them in series - one after the other - thus presenting a 16 ohm load to the amplifier. This will cause no harm to the amplifier.

If you wired them in parallel, then the effective load would be only 4 ohms, and this would cause more load on the amplifier. Depending upon the design of your amplifier, this could cause more distortion.

In closing, it is only in the 'mid range' that the impedance of the speaker is the nominal load - say 8 ohm. At 'resonance' - generally at frequencies of a few tens of Hertz, the impedance of your speaker will be a few times its nominal impedance. At high frequencies, again the impedance of the speaker will increase to a few times nominal.

Your amplifier has no 2 ohms output impedance. The output impedance will be smaller than 0.5 ohms. Put your subs in parallel. In audio we only use amplifier and loudpeaker bridging. Don't believe the myth of matching. Scroll down to related links and look at "Impedance bridging - Wikipedia".

That depends on the output impedance.

In electronic we use voltage bridging, that is a relative low output impedance to a higher input impedance. Usualy the input impedance is more than ten times higher then the output impedance.

An input impedance is called also a load impedance or an external impedance.

An output impedance is called also a source impedance or an internal impedance.