How will i know?.

When i can leave Sophie and Rosie alone together in the room while i'm at work. Their still in the process of bonding but i don't know what they should be like before it's safe enough?.

Also Rosie saw Sophie using her litter tray upstairs and an hour later Rosie used Sophies downstairs!. Is this ok?.

Sounds like you are on the way to a good relationship.

Spotz

You will know trust me - just give it a little more time.

As to them using each others litter boxes - that is normal !!!