How can one combine two wives with justice between them, and conceal the matter of the second marriage, knowing that the second wife will embrace Islam at the hands of the husband?

Acquaintance between non-mahram men and women is a door to fitna (temptation). If a woman expresses a desire to embrace Islam, she must be assisted, and this should not be delayed. If she embraces Islam for a worldly purpose, such as marriage, and then her Islam improves, there is no harm in that.

If a woman embraces Islam and you wish to marry her, and she does not have a Muslim guardian (wali), the Sharia judge should marry her. If there is no Sharia judge, then someone trusted for his uprightness among Muslims should marry her.

As for justice between wives, it applies to spending nights with them, whether they are in the same country or different countries, unless one of them waives her right. Concealing the marriage does not affect its validity.