I use EMT exactly this way.
There is NO issue within the tower, just a modest compression load and a tiny
torque.
I use the splices made for the EMT, but I drill 3/16” holes through the EMT so
the screws thread through the coupling and into the EMT.
I’m sure these will eventually wallow out, so you might want to drill an
additional pair of holes in the couplings. With an A-3 tribander, I see no
problem after a couple yrs of near daily use.
I use a centralizer at each tower joint. The ends of RG-6 cable spools fit
perfectly within 25-G, not quite a foot in diameter, I think, and make fine
centralizers when heavily varnished.
Your problem is above the tower. EMT comes in 10’ pieces, AFIK, and the
couplings wouldn’t begin to take the bending load due to even one Yagi .
You need at least two feet within the tower, leaving eight feet for the stack.
Not near enough!
The EMT will obviously stand up fine, until you have serious wind. Maybe you
can find a mast calculator that will estimate a design speed.
It seems to me that your stack is an overreach for any reasonable mast, in
terms of antenna separation, but you can get a 20 or 22 foot mast if you want
to go for it. Then you could splice the big mast onto EMT within the tower.
What you can get away with depends upon your wind exposure and how frequently
you are willing to replace the mast and maybe repair antennas.
WL
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