malternrorci fu'ivla

x1 is an absent or deadbeat co-parent of x2, the 'primary' parent of whom is x1.

x3 is 'primary' only in a grammatical sense; this designation does not imply anything about the legal or actual interpersonal relationships, expectations, etc. involved between x3 and either of x1 or x2. In fact, x3 could be a deadbeat parent to x2 too. This word can be used for the terms "baby daddy", "baby momma", etc., but only in the strictly derogatory sense; for a more neutral connotation of the same, just use "ternrorci". The co-parent need not actually be absent or a deadbeat, but some negative feeling is connoted by this word; the co-parent (rather than 'primary' parent) relationship is important to the meaning – the "mal-" seltau has broken the symmetry between rorci1 and rorci2.

In notes:

x1 is the former child of x2 by law/custom/norm/standard x3.
x1 is the co-parent of x2 with parent x3.