Swapfiles conflict when opening session in two Vim sessions

[mawkler]

When doing :OpenSession in two different Vim instances I get the following warning:

E325: ATTENTION
Found a swap file by the name ...

Despite having the following set in my .vimrc.

set shortmess+=A
let g:session_autosave = 'no'
let g:session_autoload = 'no'
let g:session_lock_enabled = 0

Shouldn't set shortmess+=A suppress this warning?