I know we discussed contact lenses for a head mounted display on one of these forums. However, the bifocal topic is new.
With contact lenses there are few different approaches to bifocals. All off them are soft ones, as far as I know. One is a bit larger than a normal contact lens, but not totally round. The lower portion is flattened, so it tends to rest on the lower eye lid. When you look down, your lower eyelid pushes against the contact lens, so that you are no longer looking through the center of the contact. The lower portion of the contact lens having the reading distance prescription.
The other technology, which I am less aware of, has the eye always seeing through both focal lengths. Essentially creating a double image. But, one of the images is out of focus, so it is supposed to be viable for the wearer to concentrate on the in-focus image. However, this obvious has a certain amount of image quality reduction.
I just did an internet search and came across this great article on the various multi-focal contact lenses, http://www.allaboutvision.com/contacts/bifocals.htm
I can also see other technological possibilities. A holographic optical element, similar to that used by the manufacturers of transparent rear-screen projection surfaces may be viable. Such a lens would accept the light from a projector coming a specific direction, while still being transparent to light from outside. I imagine the holographic lens can also have a different focal length from the rest of the lens.
Now that I think about holographic optical elements, I imagine it might just be possible to adapt the same idea to non-contact optics for a HMD. One of the challenges with optics in a HMD is that as your eye moves around, you move out of the focus point for a standard lens. A holographic lens might just be able to adapt to that by presenting the eye with what amounts a different lens as it moves around. As I think even more about it, I was reminded that holographic images are specific to certain wavelengths. But, rather than being a liability, it would allow the creator to adjust for chromatic aberrations. In other words, create one holographic lens for red, another for green, and a 3rd for blue, and put them on a multi-channel hologram.