The old Cushcraft 144-20T CP antenna (Part of AOP-1) used Gamma Match on both phased yagis, on a common boom. This wasn't the greatest antenna in the world, it suffered precipitation static discharge because of the Gamma Match being ungrounded, among other mechanical deficiencies. But it did seem to have acceptable circularity, good directivity and plenty of gain I suspect the problem may be in achieving identical phasing between antennas. A phasing problem will affect circularity.

See tuning discussion in the archive:

http://www.amsat.org/amsat/archive/amsat-bb/200107/msg00595.html