Mount one lever where you like it. Then mount the second loosely and slide it close to even by eye. Lay a broom stick or similar across both and lower your head until you can line up the broomstick with the tops of the bar. Any error will be obvious. Adjust, recheck, and tighten.
BTW- if you just mount these by eye and can't feel a difference when riding that's close enough. It's not like your arms are identical anyway.
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