they shall
That is, the descendants and successors of Ptolemy Lagidae and Seleucus, not those very personages. The prediction was fulfilled in the marriage of Berenice, daughter of Ptolemy Philadelphus, to Antiochus Theos, third king of Syria, B.C. 285-247.
6 And in the end of years they shall join themselves together; for the king's daughter of the south shall come to the king of the north to make an agreement: but she shall not retain the power of the arm; neither shall he stand, nor his arm: but she shall be given up, and they that brought her, and he that begat her, and he that strengthened her in these times.