Not a silly idea. We considered putting both girls in the queen bed when we were transitioning DD2 out of our bed. Decided though to keep the queen for guests and get a bunk for the girls (just the practicalities of our house and future plans)
When were were doing up their room they shared the queen in the guest room and it worked really well. They both slept and I think they liked having each other close.
DD1 has slept in a queen pretty much forever. She has a single but prefers the bigger bed. DS often hops in there. She's never had rails so I can't help with that. I think because they both coslept for so long, it's just what they're used to
My oldest boys slept in a double until we moved house and needed the space, so got them single bunks. They still shared a bed for ages though, with DS2 only transitioning to his own bunk when DS1 was about 5 and wanted a bed to himself.
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