I don't think I can help you in making your decision.

My first 2 started sharing a room when my DS2 was about 13 months old...this was due to DS1 constantly getting out of his bed and climbing into his brothers cot, so simple solution they shared a room, and they have been sharing ever since. When DS3 came along he had his own room until just before my 4th son arrived. So at present I have 3 boys in one room and the baby on his own. It works well and to be honest I know that they don't cope sleeping in a room on their own (only because they are not used to it). When bubs is around 1 and is sleeping well I will put DS3 in with him so I will have 2 and 2.

I'm probably leaning towards putting your eldest 2 together, depending on what sort of a sleeper your baby is would it be too disruptive to have one of the others in with it.