i think i know how to do it someone taught me(I forgot who)
but he showed it to me....
all he did was equip a crossbow plant a claymore then run far away from it
but before you run so far make sure you get hurt by your claymore by detonating it at a range that is able to hurt you and make your character fall...then when your character does the fall down...quickly switch to your crossbow by holding down triangle...then your weapon should switch to your primary BUT you are still carrying the same weapon...try it...