I colored my hair at home 1 week ago and it didn't cover all my hair. Can I repeat the process in 1 week using 2 boxes instead of 1

Yes

Coloring your hair at home can result in an even color or hair disaster. You will achieve better results if you let a professional color your hair. Yes, you can re-color your hair, but if the color still looks uneven or doesn't hold, get it professionally done.