We'd need to know what the amps are you're talking about. Solid state or valve? But as AndrewA said, whatever value impedance speakers you use (and they would need to be the same), when connected in parallel, the overall impedance the amp sees will be halved, and if connected in series, the overall impedance the amp sees will be doubled.

Solid state amps are normally happy driving into a wide range of loads, with 4 ohms being a typical minimum load for a guitar amp, and the power output of the amp decreasing as the load impedance increases. When one of these amps has a couple of output jacks, then (unless its a stereo amp with an output per channel) these will normally be wired in parallel.

Valve/tube amps have an output transformer and need to be correctly matched to the load impedance. They normally have several outputs, each output designed for a specific load impedance or combination of (similar) load impedances e.g. a connection to the 8-ohm transformer tap if two 16-ohm cabs are connected. It is very important to select the right connection or you can do serious damage to your output valves and output transformer.