I did similar last year with DDs (12 and 8). They were in bunkbeds in a small room and I moved them into a larger bedroom which had a complete makeover. It wasn't ideal for them to keep sharing but needs must and it worked out quite well.
The room is a decent double but it had to fulfil a lot of criteria - separate spaces and lots of storage. Oh and my budget was about 50p...
I used an app called Roomplanner - you can scan in a room so no need to measure anything. Then I dragged and dropped various bits of Ikea furniture in and 'painted' the walls. The girls then got to do a 3D walkthrough and were able to make choices on colours, layout etc.
What they've ended up with is a sage green room with a fairly natural/minimalist vibe and loads of storage but no actual bedroom furniture. DD1 had a daybed with drawers and DD2 has a Malm bed with drawers under. Daybed can be extended for guests or whenever DD fancies a big bed (she can convert it in about 10 seconds)
There's also a coat stand which dumps all her uniform on - quick way of preventing floordrobe.
Then there's a stack of Kallax units arranged at right angles which gives DD1 a fairly private desk area hidden behind (nice bamboo desk with pegboard above).
DD2 has a quirky Ikea desk with masses of space for craft shit.
This gives the girls own beds, capacity for guests, study areas, clothes, books, toy, craft storage and a decent amount of floor space in the middle for roller skating and cartwheels.
Visualising beforehand really helped as I knew what would fit and just tweaked design along the way!