These days Xinerama is the only mainstream tool for dual head, but there used to be others. Nvidia Twinview was one. I bought my first dual head box in 1996 with two Matrox Millennium cards (although it mainly ran NT4) and those cards later went into my dual Athlon XP machine. That ran SUSE until Ubuntu came out.

Xinerama isn't a sine qua non. It's just easy so it became ubiquitous. Maybe it's time to replace it.