This glitch can be performed on any of the girls. In order to perform this glitch, you must not be married. You can then choose among any of the 5 girls in the game that you wish to have two kids with. The girl that you choose must then marry the rival. Afterwards, start giving the girl gifts until she reaches a pink heart. Next, buy a blue feather, and make sure you already have a kitchen built (since it's a marriage requirement). You must then propose to the rival's wife before they become pregnant with the rival's child. She will say something along the lines of not accepting it, but the blue feather will disappear and you and her will in fact be married the next Sunday. Once you meet the requirements of having your child and your rival's child, they will be born. Your rival's child will be born first and your child will be born second if the glitch is performed correctly.