An important and expected part of rugby core skills.
Moviing fast with the ball rugby ball is not the same as just running.
When you run fast you will tend to use the one arm carry.
It allows high speed running and gives you a free arm to hand-off would-be tacklers.
The rest of the time you would tend to carry the ball in two hands because it allows you more easily to take options to pass or kick. I`ve called it the two hand carry
When you know what to do, do it often. Take every opportunity. Have your own ball so you can carry it when you go running. Do plenty of running, get plenty of practice.
Also be prepared. Scan the situation constantly so you`re well informed and make good decisions.
When you pass well to a supporting player you create an advantage. All the small advantages you create go to winning the game.