I tried taping two mats together for cutting purposes only, but it did not work. I think PaperPrincess has a good idea.
When I used to do drapes I had a very long table - over 8 feet long. My DH made me a nice smooth wood T-square over 8 feet long. I didn't worry so much about actal measurement, I just marked a V notch in pencil on the T-square location and used it as a guide to be sure I was cutting straight. Never heard of mats or rotary cutters! For corners, I used a big heavy 2' L square. But drapes aren't requiring the accuracy of quilting!
To square up now I lay the quilt on the floor, and measure kitty-corner both ways. If the two measurements are not equal I have a problem! If they are, I'm good.