[TowerTalk] Spacing between Antennas

[Edward Sylvester]

Greetings to the Group!
 
My friend is going to be installing a dream antenna set up for HF, on one tower.  Here's the scenario:
 
1 ea UST HDX689 89' tower
1 ea  2 ele Optibeam 80 yagi (17 sq ft)
1 ea MonstIR 6-40m yagi (25 sq ft)
1 ea 24' three inch chromally mast
 
The plan is to mount the 80m antenna towards the top of the mast and the Steppir underneath.  Obviously the more space between the two, the better.  Will 15' of separation between the two suffice, especially since the Steppir can retract all of its elements?  
 
Also, is the system beefy enough?   Your opinions are welcome.....
 
73,
Ed NI6S