I’ve used dual band wire verticals before and continue to be impressed with the results that a few dollars worth of materials and an hour or two of labor yielded. You can make this antenna with as many vertical radiators as you have room for, and use a variety of insulating materials. You can even make it quarter-wave, half-wave or a full wavelength in size! I chose ½ wavelengths for each of my favorite bands (17 and 20 meters) and because I also wanted 40 meter capability.
For ½ wavelength antenna use formula of 468 divided by Frequency in MHz.
For 14 MHz, the wire length is 33 feet long and for 18 MHz its 26 feet long. My design concentrated on a "heavy duty" version to survive the rigid winters up north and because I would run high power.
 
This antenna actually works 10-40 meters with an antenna tuner! For 28 MHz it’s a full wavelength long, on 21 MHz it is .75 wavelengths long, on 40 meters it’s a quarter-wavelength. I plan to use quarter inch rope to hoist the antenna up a nearby tree, or use my antenna raiser (see OPAR by KL7JR) for support.

In the photos below, the two coiled wires on top of the 6 inch square PVC tube (left over from an awning project) are the vertical radiators. All other coiled wires make up the ground radial system (put out as many as long as you can). I used whatever scrap wire I had laying around for the radials. The PVC pipe at top of photo is used to support the top of antenna. The vertical wires are spaced about 18 inches apart to avoid any interaction of the wire antennas to each other. I’ve seen a commercial 5 band wire vertical that used 6 inch spacing but always thought that was too close. You can decide on the spacing of your antenna, size and the number of bands you want to work. Have fun with your antenna experimenting!

Note in drawing above that antennas are spaced 18 inches apart at top.
An alternate method of hanging antennas is shown with red lines which enables
coax center conductor connected to both antennas at one point on the bottom.
Since the spacing is 18 inches at the top, there should be very little interaction if any using the alternate method. Strictly your choice.


The two coiled wires on top of the 6 inch square PVC tube in photo above,
(left over from an awning project) are the vertical radiators.
All other coiled wires make up the ground radial system (put out as many as long as you can).
Length of PVC pipe in the grass is the upper support.