12’ T&G shouldn’t be too difficult to get into place. Assuming the boards “run” reasonably straight and true. Ones with lots of knots can and do “cup”, warp and may not lay completely flat in the groves. Which can be a PITA because a 12’ board does not have a lot of flex like say a 16-20’ will have.
Most T&G will/should fit very tight together the T will not always slip easily into the G. Good chance a lot of the boards will need to be pounded into the grove. But you don’t want to pound it into place by hitting the Tongue with a hammer or the Grove if the bottom board is started with the T down.
You want to take a piece of cut-off scrap to pound on. About 6 inches long cut off the T or the G depending on how you started stacking place it on the top board in the groove or tongue to pound on. I run it through my table saw so it is about 3-4 inches wide makes it much easier to hold in place while pounding the board into the grooves,tongue and give a flat surface to pound on. I make up a bunch of these because they don’t hold up, last long with a lot of pounding if you are building a lot of stalls.
It is important that the top board is held in place tight by the grill/bars put on top of it and nailed to the header at the ceiling. The same if you are running the wall all the way up to the header/ceiling. IME the top board, like the grill/bars has to fit real snug. Boards, especially when used in an environment that is not climate controlled “move” over time.
If there is any kind of “play” from what ever is used to to keep the “down force” on the wall boards. They can and will move, cup and you may see a very noticeable amount of the tongue being exposed. This will create a "weak link " in the T&G "system. The boards can do do “spring” out from each other, horses leaning on them, the odd kick will dislodge all of your work and be an eye sore. The outer wood of the “groove” can and will split, splinter. So securing the top board with a fair amount of “down force” is pretty paramount for the walls to stand the test of time and not become an eye sore, maintenance hassle.
It is really important to “work” the boards tight into the grooves. If the broads, some of the boards are cupped they may spring out of the groove as you work your way up the wall. In this case I place a middle strapping board and screw it to each board to hold everything in place until the top board is tightly secured to the top plate, header, etc. Then I unscrew it. It only leaves small screw holes. That can be left or easily filled. We’re not making furniture nor doing detailed residential trim work here.
When done correctly T&G will stand up to horse abuse over time and rarely “pop, spring” out of place nor will the groove channels split, splinter.
Standard 2X framing lumber 2X6 or 2X8 laid on top of each others works fine also. But it must be strapped in the middle. I use thin flat bar steel for this. Drill holes and screw. For superior strength I use a piece on both side. Secure a piece to both sides of the wall and then drill the holes so the holes line up and bolt with round heads. Cut off the excess bolt and round over with a grinder.Round over the top and bottom edge of the flat steel bar at the cut off.
For a more finished look run the 2x framing lumber through a table saw to cut off the “rounded edge” left from the milling process. This will also square up the edge in a cupped board so they lay flat on top of each other.
If your lumber supply allows I like to “hand pick” my T&G boards when I pick them up. But I have a long flat bed trailer. When hand picking picking lumber show some courtesy and replace the “rejects” back on the stack, don’t make a mess for others to clean up.
If "hand picking is not allowed and a lot of yards don’t allow on large orders, time consuming and the mess factor. Ask about their return policy and replace for “rejects”. Some are fine with it others no so much. But don’t expect the yard to come and pick them up and replace. You’re generally on your own. Be realistic about what really constitutes a “reject” and what is considered reasonably acceptable all things being equal by those in the know. Lumber yards are not a “Home Depot” they cater to far more pros then weekend warriors. They will be far more receptive to you and your needs in the future if you are not a PITA amateur. The same way I feel about selling small loads of a hay to horse owners who nick-pick over every bale. I am always “out” of hay when I get calls from those types.
If the top board on a T&G wall is going to be left “floating”. A screw rod is/should be used in the middle if you don’t want the look of strapping board, or flat steel bar.
Sorry if there are any typos, grammar, punctuation mistakes or lack of clarity. No ones paying me, lol