Is it permissible for two people, one of whom is married, to embrace Islam in order to get married and remain together?

Both of you can embrace Islam, which is the greatest decision leading to eternal happiness. If your friend converts to Islam, there is no impediment to your marriage, provided he has the financial and physical capability and can deal justly between you. Otherwise, he should limit himself to one wife, as Allah Almighty says: (But if you fear that you will not be able to deal justly, then [marry only] one).