Hello, @GG0712
Thanks for your post.
1. Officially, it is suggested putting different BSP distributions to the different directory for use, since the situation you mentioned is not formally tested.
2. From my personal experience, in order for saving the size, you may use the same downloads directory, I did not test with the same sstate-cache before, but it is said from the Yocto wiki that:

You may test it and check if it is correct without issues.
BR
Chenyin