Without NVSE only one or the other may be applied. With NVSE I hear you can have upwards of 10, however this is problematic in that you have to have so many variations of each model it becomes cumbersome. Also if you feel like mucking around with Nifskope, you could add both items as a single mod by adding them both to the base mesh and using that for the one mod. Would be interesting to see someone add the bayonet as an active melee mod. Make it like a menu item that switches the rifle model into an actual 2 handed melee weapon, then back again once you take it "off".