When you trade a player you can move any eligible dual position player into the that slot and then trade a player in for the other slot. For example trade a FRF, and if you have a 2RF/FRF dual position player they can be moved into the free FRF slot allowing you to trade in a 2RF into the free 2RF slot.
When you are on the trades page any eligible player within your team for the slot you just traded out should have an S for substitute next to them allowing you to move them into that spot.
When you are on the trades page any eligible player within your team for the slot you just traded out should have an S for substitute next to them allowing you to move them into that spot.