I want to take another wife to rescue a woman who is struggling. Is this intention enough?

The intention to help is noble, but it does not replace the conditions of ability and justice, and it must not sacrifice the rights of your existing family. Helping someone in hardship can be done in many ways: charity, zakat, seeking help from welfare agencies, or connecting her with the right people. Marriage brings lifelong responsibility, not just a fix for a passing hardship. Ask yourself whether you are truly able to be fair, or whether ‘rescuing’ is just an excuse covering other motives. If you still consider it, make sure your intention is pure, the ability is there, and you are open with your wife. True compassion does not wrong anyone else. See critical wisdom.