Most shingle manufacturer's (and building codes) allow asphalt shingles to be installed in normal fashion on roofs down to a 4-12 slope. From 4-12 down to 2-12 you need to follow the "low-slope" installation requirements. Ashpalt shingles should not (actually, it's a code and installation violaton) be installed in roofs with slope less than 2-12.
That said, the typical "low-slope" installation simply requires an increase in lap on the tar paper that goes under the shingles...with some requiring a complete layer of something similar to an ice/water membrane. The membrane goes on first, followed by the tar paper...usually a double layer of tar paper. Your shingle manufacturer will have detailed instructions for "low-slope" installations either on their web page or written on the inside of the shingle package.
Some roofers also advocate reducing the tab exposure of ashpalt shingles on low-slope roofs. In one regard, it does make sense, however...
...it messes up the nailing pattern of the shingles, making them more susceptible to blow off from high winds. This is because with the reduced exposure, the nails are missing the targeted nai area and instead are being driven through the single-thickness tab on the layer below. This almost always will void a manufacturer's warranty...if that matters to you. Again...check to see what your manufacturer specifies for their product on low-slope installations.
On a low-slope, especially with your 2-12, I think you'd be better off stripping the current layer and re-applying roll roofing. If you're set on asphalt, I still advocate stripping the roll roofing. Then pay strict attention to your mambrane and tar paper layers. Don't skimp. A few extra bucks for a few rolls of the ice/water membrane is a helluva lot less expensive than on roof leak.
Also, a low-slope roof, especially with a skylight, requires special attention to flashing. Know what you're doing before you proceed. Caulk is NOT good for flashing. It will fail over time.
The reason I advocate stripping the roll roofing is to reduce the weight on your already low-slope roof, and also, most manufacturer's don't warrant their roofing products when installed as a second layer.
Have fun...and check those installation instructions before proceeding.