Welkom SuidAfrikaaner!
Standing on a tandem is something that must be praticed.
Communication is key to all tandem riding/maneuvering.
Practice first with only one person standing. Then alternate. Then try standing up simultaneously.
Keep your bodies and tandem upright. Do not 'rock' the tandem as you would your single when climbing/standing aggressively.
Practice a bit, use voice commands, and next thing you know it comes natrually.

Pedal on TWOgether!
Rudy and Kay/Zonatandem
Tucson, Arizona, USA