I use 2 rulers all the time when cutting fabric with a rotary cutter, especially when the fabric is folded. It is very important to position the cutting ruler so it is at an exact 90-degree angle to the fold. If that angle is off, you get the dreaded V bend at the fold. I am right-handed, so when I cut the bulk of the folded fabric is to my right. The cutting ruler is at the left edge of the fabric. This means most of my cutting ruler line is off the fabric, to the left. What I do is take a second ruler (the widest one I have that has a long line marked on it) and place it to the right of my cutting ruler. This way I can position a line on the second ruler along the fold of the fabric. Once I do this, I hold that second ruler in place while I butt my cutting ruler up against the edge of the second ruler. This ensures that my cutting ruler is at the most accurate 90-degree angle I can get. I will see if I can find a Youtube video that demonstrates this.
Edit: Found this video which, at about 3 minutes in, starts showing what I have described above. I have not watched the entire video so don't know if I do all the other things she does, but this 2-ruler method is what I do when I start cutting. I don't do this with every cut -- always with the first cut, and then any time I want to check on the squareness of the fabric edge I am cutting.
https://www.youtube.com/watch?v=42w5fuQmqSI