Definitely separate them before they're old enough to start developing hormonal behavior.That is generally when the fighting starts,and seemingly social hamsters go from living together in what appears complete harmony to serious and potentially deadly fighting.It is not something to take lightly,and if you wait too long you can end up with severe consequences
Regular 20 gallon tanks are not much bigger than a 10 gallon,but 20 longs are considerably longer(though only a tiny bit wider and the same height).
It isn't a matter of just being prepared and hoping nothing happens,but understanding that it is not appropriate to keep pairs and taking action right away.The sooner they're separated,the better!