Working new stitches in dbl-YO is straightforward, but it always helpful to understand more about it. There are a couple of ways it can be done.
Working multiple stitches into a double yarn over
When working more than one stitch into a yarn over (YO) or a double-yarn over (dbl-YO), always work a knit then a purl, or a purl then a knit. In Belon a k1, p1 is done in a dbl-YO, while in Tangiers it calls for a (p1, k1) 4 times in a dbl-YO.
The double yarn over increases two stitches on the row or round just worked (the row/round this stitch is worked on).
However it is possible (but not required) to also increase your stitch count on the row/round following the double yarn over (the row/round on which you work into the double yarn over). For example in Belon, no stitches are increased on the row where stitches are worked into the dbl-YO.
Regardless of if you increase the stitch count or not, you must alternate knit and purl stitches (or purl and knit stitches) when working multiple stitches into a YO or a dbl-YO. If both were worked as either purls or knits, one stitch would be just an extra wrap, not an anchored stitch. However, if a knit or purl in one YO of the dbl-YO stitch is followed by its opposite, that will create an anchored stitch.
When multiple stitches are worked in a double yarn over, they are still worked as a knit, purl alternating combination.
3 stitches in dbl-YO = k1, p1, k1 OR p1, k1, p1
4 stitches in dbl-YO = (k1, p1) 2 times OR (p1, k1) 2 times
5 stitches in dbl-YO = (k1, p1) 2 times, k1 OR (p1, k1) 2 times, p1
6 stitches in dbl-YO = (k1, p1) 3 times OR (p1, k1) 3 times
7 stitches in dbl-YO = (k1, p1) 3 times, k1 OR (p1, k1) 3 times, p1
8 stitches in dbl-YO = (k1, p1) 4 times OR (p1, k1) 4 times
TIP: As the number of stitches gets larger, work half or half plus one of the stitches into the first YO, keeping the other on the left needle. Drop the second YO and work the remaining stitches in the second YO. Adding all those extra stitches can feel a little tight, but better to be a little crammed than too loose.
Counting the Increases
As stated above, when the same number of stitches as there are YOs are worked, there is no increase in the stitch count on the row following the row the yarn over is worked on. One YO = 1 stitch.
When working more than the number of YOs then the increase is the number of stitches added, minus 2 stitches (the original YOs). For example, in Tangiers, where 8 stitches are worked in the dbl-YO, the increase is 6 stitches: 8 – 2 = 6.