All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
If you are fulfilling the due financial maintenance of each of your wives, then whatever you give them beyond this, like a personal monthly stipend or anything else, is considered a gift, which is recommended but not obligatory. Therefore, there is nothing wrong if you give one of them the stipend and deny the other. However, we do not suggest that you stop giving your wife that stipend merely because she will work and have a salary. Offering gifts fosters affection, and were you to deny her the gift after she got used to receiving it from you may lead to the opposite (less affection), especially if she knows that you continue to give it to her co-wife. Therefore, you should be wise in this regard.
Allah Knows best.