You can not use the trunk to double or even triple wield a weapon. That would be way too overpowered. The trunk is a pretty often discussed part of the Loxodon race. The creators have very carefully described what it can do. In a nutshell, it is only useful to pick up certain things - or stow them away - when you have both hands full. Anything more like that is probably going to lead to a short and firm no from your DM.