I think you are looking for :help :keepalt
:
$ vim a b current buffer is 'a', no alternate file
:b#bn current is 'b', alternate is 'a'
:b# current is 'a', alternate is 'b'
:b# current is 'b', alternate is 'a'
:keepalt e c current is 'c', alternate is still 'a'
:keepalt bd current is 'b', alternate is 'a'
:b# current is 'a', alternate is 'b'