If the combined GVWR of the power unit and the trailer is more than 26,000 pounds then you need an IFTA sticker.
If the combined GVWR of both the power unit and the trailer is less than 26,001 you do not need an IFTA sticker.
If the combined GVWR is less than 26,001 and for some reason you are stupid enough to let yourself get caught weighing more than 26,000 pounds, you are subject to being ticketed and fined for being overweight, no IFTA sticker, and likely for fuel tax evasion. Overweight and no IFTA should be around $1000. If they think you are intentionally evading fuel taxes and cite you for that, too, that'll start at around $2500.
Do not register the truck at 26,000 pounds unless that's what it weighs. Register it for whatever its GVWR is. Meaning, if the truck is rated for 11,500 pounds, then register it for that, or as close as possible, like for 12,000 pounds. If the trailer is rated at 12,000 pounds, that puts you at 26,000 pounds. Most cops will look at the door plate for the truck GVWR, and then the trailer plate, and add them together. Some will look at the registered weight, or the actual weight (whichever gets you in more trouble).
In Ohio, and several other states...
QUALIFIED VEHICLES:
The following vehicles are required to display IFTA decals and report fuel use under the IFTA program:
1. Power unit having two axles and a gross vehicle weight or registered gross vehicle weight exceeding 26,000 pounds.
2. Power unit having three or more axles, regardless of weight.
3. Power unit used in combination when the weight of such combination exceeds 26,000 pounds gross vehicle weight.
So if you register the truck for 26,000, and the trailer is at 14,000, you'll get a ticket for no IFTA sticker on a 40,000 pound vehicle. And, at least in Ohio, it'll hold up in court unless you re-register the truck properly before the court date.
Better instead to just register the truck and trailer properly, and don't go overweight. In the rare case where you are overweight, they aren't likely to ticket you for no IFTA or evasion, and will instead allow you to buy a temp IFTA or fuel permit. Getting an IFTA sticker (or apportioned plates) unnecessarily will cost you probably about $600 a quarter in paying double fuel taxes.