So my question is, if i buy a second video card (the same one), will it work on my old motherboard ?

Someting special to do or just install the new card in the slot ?
As long as those boards have a PCIe socket, you can use it. You are using RX series if you run on stock bios it's easier but if you modification your bios card, may you need to bypass a driver signature to make sure a system can read your cards.
I also use an old board on my first GPU rig. This is my first board and still used till now: