Generally one person wears the tiara for the lifetime loan
It avoids 'unflattering' comparisons between people
There is a rumour that Beatrice and Eugenie picked their potential wedding tiaras donkeys years ago. I can imagine that being a 'coming of age' thing that could be done with a Granny and her Granddaughters
I'd love to get a peek in the vaults - apparently there are jewels that haven't been seen for 100+ years
The Queen is, in comparison to some royal houses, quite stingy with loaning jewels. Meghan should have been smart and played the game
She ended up with one of the better tiaras - it's blingy and has interchangeable pieces. It also has no comparisons as it's not been seen for decades.