The resistance value of the double head anchor is the same as the equivalent lift head anchor (same SWL and same length). Indeed lift head anchors and double head anchors have the same body and foot diameter.
Like for the lift head anchor, the anchorage is produced in the concrete due to the foot of the anchor which generates a compression cone. So, the anchorage resistance depends on the concrete resistance (fck), the length of the anchor (L) and the distances to the edge of the concrete (TN) and between each anchor (TN).
The wrenching resistance values are shown in paragraph 2.1 relating to lift head anchors.