Can Thunderbird use the same Junk filters in multiple accounts?

No! You created this nice collection of filters, but Thunderbird won’t let you use it in more than one account!

The solution offered is to copy the .dat file containing those rules to the folder of another account. However, this means that you will delete other rules you may have already defined for that account (no way to “attach / add” the new rules!), and, even if you don’t mind that, from that moment on you will have to update both accounts — no way of synchronization! Wow! Yet, a better way to solve the problem is sometimes possible, but not always.

Instead of directing your Junk / Spam messages to different folders, therefore needing two sets of filters, why don’t you configure both of your accounts to use the same Junk/Spam folder? This way you can have only one set of filters in whatever account you prefer, and changes will affect both of your accounts.

To make this adjustment, go to “Account Settings”, then to “Junk Settings” of your accounts. Near the bottom you will find the option “Move new Junk messages to”. Just select the same folder in the Settings of your accounts, and enjoy!