Rather than taking a second row you can shoot in portrait mode. I know of two issues with multiple rows.
- Unless it is during astronomical night light level changes during the time it takes to get all the frames can produce complex gradients across the field.
- Without a panorama head it's difficult to get the frames precisely aligned.
Given those considerations, with two rows you probably do best if you shoot the frames in the order:
2 3 6 7
1 4 5 8 etc
to minimize the gradients and is hard work manually.
As for using a fisheye like the Samyang 8mm, if you need to stitch you'll need some pretty smart software to deal with the distortion. While it won't affect you with the K-5, the Samyang 8mm is an APS-C lens, as the vignetting using one on my Sony a7s proves.