When i tried a combination like this, i feel a STRONG potentiation but .... only from the codeine's effect !
If you want to take a opiate trip with codeine and dxm, just 2-3 pills of dxm sufficient but If you want a DXM like trip, i advice you to diminish a lot the codeine !
EDIT : i have just thinking that 2-3 pills of DXm rougly equals to 100mg , ^^
So i think you will appreciate a fine time (don't do too muchDXM while on codeine, it's not needed)