backup for imessage with multiple mobile numbers

Hi,

Since imessage allows you to have multiple mobile numbers linked to an account/1 apple ID, i wonder how the backups would be like? say if i have 2 iphones and they're both signed up using the same apple ID but they have imessage that send/receive messages from 2 different numbers. Will imessage be able to backup chats of the two different numbers or will it get overwritten? Is it safe?