Indeed.
The distance depends a bit,
Normally a halve wave length or just beyond that is adviced.
I took a random 3el yagi:
On its own the antenna would have 7,6dBI (which is what is to be expected)
If we stack 2 of them vertical we will find:
4 meter distance will add about 2 dBi
5 meter distance will add about 2,6 dBi
6 meter distance will add about 3,1 dBi
7 meter distance will add about 3,4 dBi
8 meter distance will add about 3,3 dBi
As soon as you go beyond 6 meters large side-lobs will become appearant.
(you dont want that).
And after 7 meters the gain advantage will drop again.
So for overal best antenna pattern..5,5..6 meters would be best, but anything beyond 4 meter is worth the hassle.
it will look like this:
View attachment 72227
I can imagine the groundkit will not help in a lot of cases, but on the other hand it could help in some others (due to that common mode currents...((CMC)).
In anyway, i do find it expensive for just a couple tubes.
Im a stubborn guy i guess, i cant escape the strange feeling when i see an antenna with a additional "add-on option".... I mean... who sells an antenna which supposely isnt optimized in first place ? I find that odd
If i were you, would first install a RF choke (4 turns RG213) 1 meter below the feedpoint. Do not tape it the mast but leave it "hanging". Tape the coax just above and below the RF Choke to the mast ..so there is some tension on the RF choke in the way you force it away from the mast, a couple inches is fine.
Also make sure you ground youre mast, and ideally connect the outer braid of the coax to that groundpoint too.
You could make your own groundplane kit by just installing a couple aluminium tubes with u bolts at the feedpoint. Short tubes will do...say 4x1 meter or so. If you do that you can install the RFchoke directly at the feedpoint. (again dont tape the RF choke to anything ...it needs to be "free").
That might help to deal with the possible CMC mentioned
The decision of buying a vertical or not is one you have to make on your own. If it truly is only local work what you want... spend money in antenne heigth...and well yes a vertical .. but first (antenna tip) height...that is priority nr 1.
I have an 18 meter mast and i just have a wire hanging next to the mast:
This one: extended double zepp
That beats any 1/2 ..5/8 wave all the time.
Im a "DXer" myself, its easy to get hooked (hint hint), and without a doubt a horizontal beam will open a new world for you.
Kind regards, Henry