I agree with the last two posters, that is, use high gears while standing and stand alternatively. We can stand at the same time with no problems but I can usually stand for longer than my stoker.
Here is what we do:
We go high cadence, at least 90 rpm, for as long as possible. When the end of the climb is in sight, we shift one or two gears up and I stand. Then my stoker gets up for about 20 to 30 strokes and sits to catch her breath. I remain standing until the top or very near the top with my stoker finishing standing and strong. It works for us.