If you invite Montaron and Xzar into your party, they will insist on going to Nashkel as soon as possible. They agree to a quick detour to the Friendly Arm Inn area, but will become increasingly agitated as time passes if you do not go there. They will eventually leave the party if you stray too far from their goal for too long. It is not necessary to complete any quests in Nashkel to get them to stay. Just visiting the area once is enough. After about 15 days game time without reaching Nashkel: