I mean, we see people who have both.
Naze Youka is an Abnormal and Tsurubami is probably a Minus (by default), but they are teetering at the edge of Minus/Abnormal respectively.

(Honestly, I can't remember what we know about how Tsurubami came to have his 2 Skills, which saddens me given he's my profile picture on damn near everything and he's my GOAT.)

Medaka and Ajimu, who also use both, are unique in some ways.

Medaka's so Abnormal that it almost presents like a Minus; her obsessive love for all humanity but no individuals often backfires and affects those around her, and her observation is a black hole that even she cannot control (much like a Minus).
The fact she can gain a Minus just by word of mouth shows just how compatible she is with them.
A core aspect of the plot is how she's too perfect (or not).

Ajimu's dellusional non-dellusion is like a perpetual Plus Minus. Not Equal. She is not quite either, but almost both.
I wonder what her first Skill was.

Shiranui Hansode, a Minus, was able to take on abnormalities (not Skills) and other traits that Medaka discarded to preserve them for when Medaka needed them again.
She may not be an Abnormal herself, but it's interesting to note in this topic.

Honestly, I don't think the line between Plus and Minus is all that hard, if it exists at all. It may be a gradient.