The wifey and I have over 3,500 miles and two years experience on our tandem now.
I can stand at any time while riding and keep firm control of the bike. In fact I have to stand up periodically to wake things up (but that's another issue).
My better-half has experimented with standing sometimes when I stand. At first she didn't feel strong enough to stay up but for only a second or so. But as time goes on she has been getting stronger and can stand for up to a minute now.
Our biggest problem with standing is we tend to take up a whole lane when standing. We don't wobble but we do drift left and right. It is pretty hard to "Hold our line". Other than "Practice, Practice Practice", does anyone have any technique suggestions? I try to tell her to keep her upper body quiet, (relaxed on the handlbars) and I do the same on my bars.