Women who agree to be a 2nd, 3rd or 4th wife do so for a varying amount of reasons. Some choose to completely be desensitised to the first wife so that it doesn't bother them as much and that is how they cope. Others just get on with it.
If you are in anyway desiring a second wife before or after marriage you should communicate this ASAP to the first wife, the more of a surprise it is the more shocked she will be.
Its a test for everyone involved in the marriage, I would find out if the first wife wants to know the second one or not and go from there. Just don't string the sec