If you do it in series (as opposed to parallel) then the resistance load should be double (two 8 ohm loads in series yield 16 ohms) and the resultant power/volume should be half. If you do it in parallel it would be half the resistance and twice the power/volume. The changes with load can affect the tone somewhat but whatever.
Solid state amps work as you describe if their power supply is up to it - eg a solid state bass head where the output devices are directly coupled with the load. Tube amps like Marshalls have switchable transformer taps on the output - typically 8 Ohm or 16 Ohm - and power output is more or less the same on each. You can even put 4 cabs in a serial/parallel setup and get a 6dB drop.