Try it out, and do a process of elimination... if it works, then no need to get twin beds. If it doesn't, then you do need to get separate beds for them.
Yes, they may play, fight, kick each other, keep each other awake etc. It depends on the personality of the children. MAIN thing is you want a conducive PEACEFUL sleep situation so that they DO fall asleep well and don't interfere with each other.
With my kids, my girl likes to sleep next to her little brother sometimes...BUT, it does not work out because one will want to play and the other one is tired and wants to sleep... then one gets very fussy and tantrums and it turns into a battle. So, for us, they have to be separate. And, their sleep patterns and wake times are different too.
Also, as they get older, they may want their own beds, or not. So it will take a period of time for them to adjust to sleeping together... if it is amicable for them.
If it doesn't work out with them sleeping together... I would not force it. Some kids, or even adults, just have certain ways they can or cannot fall asleep. Just talk with them, tell them to try it out... but encourage them to be able to express themselves about it too. Then, if it doesn't work out... they will know that they will have the option to have their own twin or bunk beds.
Hopefully the will sleep well, then hooray! And at least for a few years, you will not need to get other beds for them until later.
Good luck!
~Susan