question about borders
 

Is there any reason other than mitering issue, that one should not sew first and second borders together before attaching to quilt? thank you

You would have to be sure that the second border is longer to allow for the first border on the top, because they both do not meet at the same place. I know that it is a bother to have to measure through the middle at every border. I don't know there might be a way to do it and maybe there is someone out there that might know how it's done but i don't how.

A rather interesting question. You could have some interesting design elements by doing it this way. However, it will not save sewing time or thread because you still have to sew the various sections together. Let us know what you decide to do and what you think about the process.

I sewed both borders together, attached them to the quilt and mitered the borders one time. Boy did the corners come out wonky.

Y seams come to mind like you could only sew so far like partial seams. Often simplest way is not only best but less frustrating. Check out the different applications of borders on Yu Tube

Nope, no reason at all, Just need to figure out how to handle the corners. You can miter them or use a cornerstone block.

I have sewn them together first, then treated it as one border, pressing the seam open helps a lot. Mitering was just the same as with one piece.

I have done this. As long as you have a plan for the corners (mite ring or cornerstones) there is no reason not to do it.

The big advantage of combining borders before attaching them to the quilt is that you aren't dealing with the weight and bulk of the entire top each time; I find it easier to handle strips than the whole top, so I like to attach them all in one go.

I would only do this if I was mitering borders

If you don't miter the corners and just sew the joined borders on straight, it looks weird to me because the border colors won't line up. The other option to avoid that is with a corner stone.