This depends entirely on what a "secondary decker" is. Attack actions and Sleaze actions can happen almost independantly of eachother in various situations (except where Crack File comes into play).
If your role is to cover for the normal decker for something like... "Hey, I need to search the host for the file, can you mark and control cameras and doors while I do this?" then Sleaze would be the way to go. You are there to do the light lifting and ditch if things get heavy/heated.
If you're backup for if things go badly, then Attack can be the way to go. "Oh shit, I got spotted! Senko, get in here and help me fend off the IC while I finish the job!" Alternatively, you can be the overwatch guy. You're staying in the meat, using your Attack to brick things and cause mayhem and take out potential threats and screwing enemy communication while the primary decker works their magic on the matrix or in the host.