You can buy single-sided power meters that measure power from one side, usually the left, and then double the reading to estimate your total power output from both legs.
A single-sided only measurement means doubling a single leg's power may not be a fully accurate representation of your power, but it can mean they are more affordable.
It may be worth checking if you have significant imbalances before opting for a single-sided meter. Note a 48/52% balance between legs is common.
Combined power meters effectively combine the power from both right and left legs and do not measure it independently. An example would be an SRM power meter.
Note that these systems, although accurate, will not differentiate which leg the power comes from.
Left/Right side power meters, or double sided power meters are found on more modern and more expensive power meters that have gauges in multiple locations, such as pedals and some crank-based units.
This can be useful in establishing if you have an imbalance (one leg much more powerful than the other) and for working on pedalling technique. It can also be used if you are recovering from a single leg injury.
Dual sided measurement is limited to power meters that measure power in more than one location, such as pedals, but also more expensive crank-based units, like Rotor 2InPower.