While not 100% answering the spacing with HF overtly the concepts should give you an idea.
STACKING DIS-SIMILAR YAGIS AND LOOP YAGIShttp://www.directivesystems.com/STACKING.htm
As a point of thought my 6m and 2 m antennas should be about 10-12 ft apart. The 432 should fall closer to the 6 than the 2. You can review this article and get some ideas on how to calculate for your needs.
Hope it helps.