well i was going to use the spandy foam to support it more it would have no way to move in any way...its a 18" sub should i get a box wider or get one 20" ...i had some wheel boxes my wheels came in but they got tosses
if you plan to insure it, UPS and FedEx both have a minimum of 2" packaging material on all sides for it to be covered. you say 18" subwoofer and the box is 20", they won't cover it. You can pay for as much insurance as you want but without the 2" on all sides, they will tell you to **** off...
as for boxes, I like to use toilet tank boxes... they are THICK and you can usually get them for free from home repair type places...
its always cheaper to ship in 1 box but not always safe. you will get them shipped to you in 2 seperate boxes. personally would wrap each motor really good and divide a box and ship both motors together. then ship both baskets together in another box.
ALWAYS put insurance on shipping a item. even if you package correctly they can break it, it covers ur a$$ for a little more coin...