Using Speaker Wire to extend Bi-Wire Cable to Speakers

Can I hear some thoughts about using regular 18 gauge speaker wire to extend my prefab bi-wire speaker cable by a foot?

I have two speakers sitting on opposite sides of the fireplace so both amplifiers have to sit on one side of the fireplace or the other making my prefab cable short by one foot. The prefab cables have heavy duty spades on each end w/four spades going to the speaker. So can I hear thoughts on using regular speaker wire to extend the cables and which end should I extend-- the spades going to amplifier or the four spades going to speaker? I plan to solder the 18 gauge speaker wire to the spades..