tags-add-tables

tags-add-tables is a variable defined in `etags.el'.
Its value is

ask-user

Documentation:
Control whether to add a new tags table to the current list.
t means do; nil means don't (always start a new list).
Any other value means ask the user whether to add a new tags table
to the current list (as opposed to starting a new list).

You can customize this variable.