You have to miter the borders for it to look right

I sew them on seperately UNLESS I am mitering the corners, then I sew them together and sew on as one. Either way will work. 99% of the time I cut down the selvage, not across from selvage to selvage. That also will effect if it is wavy. Measuring and pinning correctly also effects how straight or wavy your borders will be.

I'm new to this, but I'd put them together first. That way you only have to deal with the bulk on the machine once.

I've always done one border at a time. Didn't even think about doing both at once. Some of you said it might cause the borders to become wavy if done together and other say it doesn't. Now I don't kinow what to do with my quilts. I think I will try to do two borders at once and miter the corners. I have a ruler for mitering. Hope it works.

I've alwasy done them one at a time

I would do them one at a time unless you want the look as illustrated or you want to miter the corners. You might also want to size the width of your borders using the Fibonacci number theory or golden ratio. Measure the width of your block and multiply or divide by 1.61. example: 9 inch block (division process) - 5.5, 3.5. 2.25, 1.25, 0.75 Any of these widths will give a balanced look. Just keep dividing the answer by 1.61 and round up or down to the nearest quarter or eighth inch as desired. Look up Fibonacci or Golden Ratio for more information.

I do mine seperately

I do them one border at a time so that I can do a herringbone effect in the corners.
I think it might also impact any stretching that can occur over a longer strip of fabric.

I do 2 long sides (3" in this case), then the short ones.
Then I do the two short ones (6" wide in this example), followed by the two long.
I hope this makes sense!

what a great idea. And you only have to do 4 miters instead of eight. Brilliant !!!

Both ways work fine but sewing them together aids in making mitered borders if that is what you wanted. No wavy if you measure right and use a walking foot or pin it well.