I *think* you are looking for `:help :keepalt`:

    $ vim a b       current buffer is 'a', no alternate file
    :b#             current is 'b', alt is 'a'
    :b#             current is 'a', alt is 'b'
    :b#             current is 'b', alt is 'a'
    :keepalt e c    current is 'c', alt is still 'a'
    :bd             current is 'b', alt is 'a'
    :b#             current is 'a', alt is 'b'