The first item multiplies the ability by 1.5, and rounds down to get a whole number, and the second item does the same starting with that whole number. So a Ring of Dark Magic and Guinevere affect a Dark Magic skill of 13 to convert it first into 19 (19.5 rounded down), and then convert the 19 into 28 (28.5 rounded down). An initial skill level of 12 would give you exactly the same result, whereas if you had an initial Dark Magic skill of 14, you would end up with an effective skill level of 31.

You get the maximum benefit when the starting skill level is divisible by 4.