Hmm, interesting question.

In 1975 the sequel to Odyssey (1972) was called Odyssey 100, so that's probably the first "system sequel" like PS1 to PS2.

The RCA Studio II (no RCA Studio I) did have some I and II titles in their education titles (TV Schoolhouse I & II), as did the Fairchild Channel F with their Math carts (Math Quiz 1 & 2). These sorts of titles seem more like "paired" titles rather then a prequel-sequel sequence. IE they seemed designed and manufactured parallel.

However, I think the first sequel on home system would be Speedway II (1977) on the Atari VCS aka Street Racer. Speedway II (the Sears name for an Atari cart) was probably named after a dedicated console which had PONG and race car games on it, that Sears had distributed in 1976. So, the first sequel on an home videogame system could be considered a cross generational system title!

In the arcades, per KLOV.com, SeGa released Pong Tron and Pong Tron II in 1973, that may be the first arcade based sequel set, unless you count PONG Doubles (Atari 1973) as a sequel to PONG (1972).

Anyway, my 10 (two) bits.