It depends on what the amps are rated at. The amp specs should give the rated power output at a specific cabinet impedance, i.e. “500W at 4 ohm”. If it doesn’t, the power rating will be for the specified minimum impedance, which should definitely be noted. The minimum impedance will probably be on the back of the amp as well. Note that some amps may have higher minimum impedances (8ohms, etc).
If you are driving an amp rated at 500W at 4 ohms into an 8 ohm cabinet, the amp will only deliver 250 watts to the cabinet. Driving the same amp into two 8 ohm cabinets in parallel will deliver 250W to each, or 500 total. Driving the same amp into two 8 ohm cabinets in series will deliver 125W to them.
See this topic:
this is discussed in detail there.