Two speakers at 8ohms each will give you 4 ohms if wired in parallel or 16 if wired in series.
Now if you wire two speakers in parallel, the wattage of the amp is shared equally.
Lets say you have two 100 watt speakers, you will get 200 watts capacity, so you will be fine with your 150 watt amp.
If you use two 75 watt speakers you will have 150 watts, and will be operating close to tolerances. It still should be fine tho.
If you use two different wattage speakers, you need take into account the wattage going into it. The best way is to double the wattage of the smaller speaker. Lets say you have a 100w speaker and a 50 watt speaker, you really have a 100w cab, because putting more than 100w into it will risk blowing the 50watter.
Basically, you need to have speakers that can each handle at least half the wattage of the amp, so two 75 watters should be ok, but i prefer to run my speakers so that each speaker could potentially do the job on its own...so if it was my rig, i'd go for two 150watt speakers, giving a maximum rms rating of 300watts for the cab. Plenty headroom, and the spakers will run nice and cool whatever you throw at them.
