The Se5 and 5a is able to fire both the Vickers and the Lewis on the same turn. It is one roll to hit and then 2 separate rolls for damage – one for each of the guns.
Correct, they both may fire, 1d6 to hit, separate jams, then for damage it is the single deck table 1d6 for the vickers and 1d6 single wing table for the lewis.
You may NOT reload the Lewis & fire the vickers in the same turn though…