if they are the same brand and model, I would not object much, but speaker impedance is very frequency dependent and commonly varies over more than a 2:1 range. It is not unusual for an "8 ohm" to present as low as 4 ohm or as high as 16 ohm loads over the audio range. It is possible to underload an amp as well as overload one. And damping factor goes to heck ... rather than having a sub-ohm driver, each speaker has that plus the series speaker.
I'm with the poster who recommends another amp ... not really expensive in low power consumer grade stuff. (We're not talking the Crown type stuff here). Mostly for damping reasons I'm loathe to recommend series connections.
At reasonable listening levels, the 2 ohm load on the 8 ohm rated amp rarely causes any problems. And with the 40 feet of #24 wire so often used ... another 2 ohms per speaker right there.
The problem with this is it doesn't matter when both coils are electrically the same, if both coils are 4 ohms they are electrically the same, if one is rated 100 watt and the other 10 watts, this would be like series a 100 amp fuse in-line with a 10 amp fuse, of course the lower watt coil would open first if the peak wattage of it is exceeded. but again both coils will electrically see the same frequency and wattage. 8 ohms is 8 ohms. Is this a good way to run a set of speakers? No, and normally I wouldn't do it. but in a pinch I have, like I said to keep a show going.
More speakers are ruined by an under powered amplifier than overpowered. Seems counter intuitive but the overpowered loudspeaker's distress is quickly recognized and countered by turning it down - uh, in most cases. It is the loudspeaker that is being driven by an amp that cannot deliver what the loudspeaker wants when the operator wants it to go louder that is in trouble. Think of the sine wave. The loudspeaker is being driven in and out like the wave is going up and down. In an underpowered situation the peaks of the sine wave are cut off the speaker stops traveling and holds either out or in for the duration of the plateau (that is the duration of the "clip"). This just sounds muddy and it not as quickly recognized as a damaging sound likes being overpowered is especially in a noisy environment. When this happens the voice coil does not move properly and the heat builds up and poof the magic smoke gets out and the loudspeaker won't work anymore. Great for special effects though and guys in the repair business.
MacG
Very correct statement, clipping an amp is one of the biggest reasons of speaker failure, and especially high frequency drivers.
A speaker that has a rating of 100 watts RMS, will have a rating of 400 watts peek. this is 6 db over rating, this 400 watt is a very short duration peek rating.
For every 3 db you increase you effectively double in power.
Clipping an amp is driving the amp to produce a non sinusoidal DC output that is 6 db over the rating of the amp, remember this amp is now sending a 6 db "DC" voltage to your speakers, that would mean if we were to drive those 100 watt speakers with a 50 watt amp, and you turn it up to the point of clipping, you are effectively sending a 200 watt DC signal to a coil that can only handle 100 watts continuous RMS.
So you always allow a 1.5 to 1 head room over the rating of your speakers, to drive any amp into clipping is just shear madness.
This is the science behind blowing speakers with a amp that is two small.