I think that this is the age when most siblings should be separated when they are sharing a room.
Reasons why:
He is starting school, he will have some minimal homework, he'll have school papers that she will get into, he will want to have friends over after school to play, they won't want to have a girl hanging around, they'll want to go in "his" room and play boy toys.
He is 5 she is 2. Surely they have different bedtimes....he is old enough to stay up later while she still needs to go to bed earlier. So what good does it do for them to share the room? They can't use it at the same time for anything more than storing their clothes and furniture.
She cannot be around toys that are for ages 3 and up. She is at risk of choking to death if he even has a hot wheels car on the floor. She may be good about not putting stuff in her mouth right now but it is a 2 years old nature to have stuff in their mouth. This alone is reason enough she needs her own room in my opinion.
Even if the toys are not in their room and you have the toys in the family room he should not have anything small enough to fit through a toilet paper roll anywhere near the floor where she could reach it. That makes it hard on him to be at his developmental stage.
I think that HUD has their guidelines for well researched reasons. They do not allow different gender children share rooms at this age. They have struct guidelines for this. When I was President of the Board for my local area Habitat for Humanity we had to make sure families that applied and were accepted had bedrooms for each child, they could have kids share a room but they had to be very close in age plus same gender.
There are exceptions to the rule though. My friend had 4 children. A preteen girl, a tween boy, and 2 year old twins, a boy and a girl.
The largest apartment the complex had was a 4 bedroom. The twins shared a room due to the age difference in the same gender sibling. The best situation for her family was to put them together at that time. But when she signed the lease they told her if her family was still this dynamic when the twins reached age 5 she would have to move out due to HUD regulations not allowing them to share a bedroom after that.
So, I think the idea of them having their own rooms so they can each have their own space is a good idea.
If a younger sibling comes along then the situation can be thought out more but for now I vote they have their own rooms with their own toys in their own room.