It depends on what power the amp is going to put out.
If it's a 600 watt amp, then it could send 300 watts to each of the cabs. 300 going to your 600w cab & 300 going to the 150w cab (which is obviously more than it can handle, but if the amp & ears are used sensibly, then it can still be used).
Think of it like having 2 balloons & a tank of air with 2 pipes that will split the air evenly. One balloon being able to take all that is in the tank & the other being 1/4 the size.
Both balloons can be used if you only turn the air on partially, but the small balloon isn't capable of taking 1/2 of what is in the tank, so would burst if the tank was opened fully for any amount of time. The big balloon would still take it's fill.
Hope I'm not just talking wind here!