A one trip whipstock system includes both a combination mill and associated whipstock. The one trip combination mill comprises an upper cutter and lower cutter. The cutters are joined by a cone that widens towards the upper cutter. A whipstock is connected to the drilling sub by shear bolts. A guide lug on the whipstock faces the drilling sub between the lower cutter and upper cutter for riding on the cone when the drilling sub is advanced along the whipstock. Preferably the shear bolts connect to the lower cutter. The lower cutter has smaller gauge than the upper cutter, to such an extent that the relative gauge sizes of cutters permit the upper cutter to exit casing during drilling before the lower cutter. A watermelon mill and a string mill above the upper cutter may be provided. To reduce outer diameter wear on the upper cutter, it has a square leading edge.