kayak stacker You could use a simple yak stacker, basically just an upright bar or U-shaped element that holds the boats up on their sides. The kayaks stick up a bit higher than if they were carried deck down or hull down.
You could probably get both boats on one side of the rack and have the other side free. Use individual belly straps for each boat if you do this and have both boats on one side of the stacker. Don’t put one set of straps over two boats and expect it to hold.
racks Doesn’t really matter. Any of the brands of racks make ones that’ll mate with factory lenghtwise bars and two boats can easily be carried side by side sitting on their bottoms, no biggy, do it all the time. If you need more than two boats, then you need to stand them up on their sides.
Cost I take it that you want to do this as cheaply as possible to save for the trailer. There is a product available at building supply stores called Unistrut. It is a steel track that accepts a lot of fittings which are also available at the stores. It comes in 10’ lengths and 2 thicknesses. You will probably want the 7/8" thickness. You can U-bolt it to your racks. It comes in 10’ lengths and costs $10-$12 a stick. One stick will give you 2 60" crossbars.