The advice about getting territorial is a good one.
So that didn't go as well as I had hoped it would. Instead of letting the younger budgie out I first let the older budgie out. She flew straight to the younger one's cage and they started acking at each other. So then I let the younger one out hoping that would calm them both down. They both landed on the older one's cage. I gave them each some millet and for a while they were just happily munching their millet.
Then the older one got bored and started snacking on the younger one's millet spray, which she seemed to allow but then the older one started pecking at her tail and backing again. They chased each other around for a few minutes before the older one flew back into her cage. I closed the cage door so the younger one couldn't follow her.
The younger one was determined though to ..god knows what..make a connection, make friends, establish dominance,...I really don't know what. Anyway she insisted on crawling around the outside of the cage and the older kept pecking at her.
I've read in the sticky on here that a bit of acking, chasing and bickering is normal but I was still worried. They didn't draw blood yet but I didn't want to let it come to that so I caught the younger one and put her back into her own cage (took me a while). Anyway, they quickly seemed to calm down again and thankfully the younger one doesn't seem to hold a grudge because I caught her. I pacified them both with millet and they seem fine now.
They are both in the same room but I think from tomorrow I'll just place their cages next to each other rather than letting them meet.
I think I really need to somehow sort out the neutral ground issue. Maybe I can let then older one out first, then cover her cage, then bring the younger one into the room, and remove her cage from the room..