Thomas,
The idea is not to have conductors running through the near field of the antennas which are parallel to the radiating elements. This is a factor with crossed Yagis such as the KLM, M**2, and similar units especially, since they need to be supported near the midpoint. A helix or dish antenna is usually mounted at the end. Some people have used wood cross booms, but of course there are weatherproofing issues.
That all having been said, people have used conducting cross booms with great success. Part of the secret is to rotate the antennas so that all the elements make an X rather than a + so that both sets of elements are equally effected. I may be in error, but I recall Roy Welch, W0SL, did some studies on this several years ago. You may be able to find some information in the archives.
Alan WA4SCA