Are you saying that they are different fabrics?? If so, it is best to miter the border. I usually prefer to attach my side borders on first. I remember hearing some rule to which side of the border to attach first, but I thought it was nonsense so I did it my way. The reason I do this is because I make mostly lap size quilts and since you will need to piece the top, you might as well have a longer piece to sew on. I also cut my borders the length of the fabric and cutting them this way, makes them closer in length. Hope this makes sense.