When you log into the same account on 2 devices and use it simultaneously,
what will happen?
On 1st device, you add 3 words to word book A
At the same time, on 2nd device, you change your active word book into book B,
and you save 5 words.
What happens?
Our server may get confused,
as it doesn't understand which data you want to save.
It may choose randomly.
So even though you can use your account on multiple devices,
it's not advised that you use them at the same time.