Found this site which gives a few technical details about stacking, but it's for "like" antennas.
If you don't have much information, check the vertical stacking distance that the manufacturer recommends for two of the same antennas. This distance is the height of the capture area!
The minimum clearance distance (for antennas on lower bands, same polarization) is one-half of the stacking distance for two of the same antennas.
Example: The manufacturer recommends that you stack two identical 144MHz yagis 10 feet apart. That means that you shouldn't mount one of these yagis any closer than (10 / 2) = 5 feet above a lower-band antenna such as a 50MHz yagi or HF tribander.
If you don't even have that much information, for long yagis you can look at the performance table by VE7BQH. That gives recommended stacking distances for a comprehensive range of 144MHz long yagis. If you can't find your exact yagi design, you should be able to find something comparable as a guide.
I have never heard of antenna stacking. what is the benefit of doing this? You have peaked my curiosity and now I want to know more about this. So if you can direct me in the right place that would be good. I will check out the link above as well.