On Sat, 2 Jan 2010 09:08:16 -0800 (PST), mr1956
wrote:

I am trying to stack two Yagi antennas to form a ground station to
receive GPS data from an experimental rocket.
(...)

The topic of model rocket telemetry communications was previously
covered in this thread:
http://groups.google.com/group/rec.radio.amateur.antenna/browse_thread/thread/c158b265a041f9b6/
While it doesn't answer your question about antenna stacking and
combining, it may offer some insight.

There was also this thread:
http://groups.google.com/group/alt.internet.wireless/browse_thread/thread/108522ee81505069
which involved model airplane control and tracking at 2.4GHz which
might be of interest.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558