WorzelG, congratulations on getting a Syrian for your two children. I know that they will absolutely love them. What a lovely and memorable Easter/pre-Easter gift for them.
As far as them sharing a ball or temporary home while you are cleaning their permanent home, it should be fine so long as they never are in them at the same time. It is also extremely important to make sure to remove the scent of the other hamster in order to reduce the level of stress. This is due to Syrians being solitary animals.
Have fun with your furry friends when you get them.