It does look like play but Luma is probably still deferring to Rosie's puppyhood as older pack dogs tend to do that until the little one needs to be taught a lesson. It could always stay this way with Rosie calling the shots or totally change once Rosie is older and Luma feels she needs to re-assert herself as leader. I would let them drink from each other's bowls as long as it doesn't cause discord as dogs seem to delight in sneaking food and water from the other's bowls and stealing toys and chewies. Just watch for resource guarding and step in to stop that if one or the other of them gets too mean about it. Let them know their boundaries and that you are there to enforce a certain behavior standard if either one starts harming the other one or one begins to get skittish, regresses into early shyness or avoidance.
Those two girls are just adorable and I'll have to say I envy you having these two precious little Yorkies. It must be so much fun and a laugh a minute most days. Work training each in obedience separately and you will have two well-behaved little ladies - most of the time! haha - no dog is 100% well-behaved, thank goodness. Nobody wants a robot for a best friend!
