Allow me to explain.
If someone is dual welding on his front bar and has a Sword and Shield back bar, he shouldn't have to use two different main hand weapons. He should just have to put an off-hand to replace the shield in his front bar and when the swap happens, it leaves the main hand weapon there. Shouldn't have to farm two of the same main hand weapons if you want to use it for both.
If you have the same type of front/back bar setups like Bow/Bow, 2-hander/2-hander, Staff/Staff of the same type setups, you should be able to use keep 1 staff between both bars and it keep it equipped when the bar swaps and the space it pulls up is empty.
There is no point in forcing players to farm for another weapon of the same kind just for both front and back bar setups, bag space is already at enough of a premium as it is and if someone accidentally leaves a space open, what possible purpose would it serve to swap to bare fists?
Give people the option to replace the weapon just by putting one in the slot for the appropriate bar but leave an option where you can just leave the slot empty and it will keep the weapon in that hand from the other bar when the swap happens.