Along the lines of Professor Dhadphale's suggestion for vekurañja as coming from Skt. dvaikulajanya, there are expressions like dvaimātura, ṣāṇmātura etc. The term dvyāmuṣyāyaṇa refers to a son with two fathers, one legal and the other biological. The sons born out of the so-called niyoga "levirate" are described with this term.  The term kuṇḍa used in the passage has a meaning of "out of wedlock," but having a dual connection, suggesting something unnatural, illegal.   So dvaikulajanya sounds like a very possible source.  I wonder if there is a northern Sanskrit āgama version.  I read from earlier messages that the non-Pali versions translated into Chinese show that this term was eliminated from the text, possibly being considered some sort of an error.