Recording L-R power with a bike computer

Not all cycle computers that record power data can also record left-right balance data of dual-leg power meters. The good news is that most do, and the only two of Garmin's extensive range that lack this capability are the Forerunner 310XT and the Edge 705. (Source: FAQ) Clearly these are two fairly old models, so you should find that any power-capable bike computer that came out since 2012 or so does indeed track power separately for each leg, when the power meter supports it.