I remember having a number of Mastertronic tape games for the Amstrad CPC 464 that could be used on a Spectrum too by using the other side of the tape. These were called flippys I believe.
Was this feature unique to Mastertronic?
Also, does anybody know if any other 8 bit computers were similarly paired.
This feature wasn’t unique to Mastertronic or to the CPC/Spectrum pairing. There were quite a few publishers and pairings, both on cassette and on disk. One example of the former is Mirrorsoft’s UK release of Boulder Dash, with one side for 8-bit Atari computers, the other for Amstrad computers. A famous example of the latter is Epyx’s release of Ball Blazer, with one side for 8-bit Atari computers, the other for the Commodore 64 and 128.
For both cassettes and disks, this works because the computer reading the medium only cares about one side, and the medium has two. So a cassette can have content stored using one platform’s format on one side, another’s on the other; the same applies to 5.25” disks (and I imagine 8” disks although I’m not aware of this feature being used there).
This wasn’t limited to 8-bit computers either; some Infocom releases were published on flippy disks for IBM PCs and Apple IIs (e.g. the 4-in-1 sampler). Quite a few demo disks were available in combined form for the Atari ST and Amiga too (but not as flippy disks — 3.5” disks can only be inserted one way). On platforms with dual-sided disk drives, the “side” for that platform has to be formatted in such a way that the other side isn’t used. Combined disks for platforms which both use both sides have to have a format where the first side is readable on both platforms, but skips the rest of the side for one of the platforms; see this video for an example with a combined Atari ST / Amiga disk.
Later on, it was also common to see multi-platform CD-ROM games (for Macintosh and PC).
One unusual example is the data cassette for the videogame adaptation of Trivial Pursuit. This contained the questions and answers; the publisher also sold expansion packs with additional sets of questions.
The data cassette in my copy had two sides. One side was recorded using the UniLODE system, designed as a "universal" tape format that could be read by any supported 8-bit computer (Spectrum, CPC, Commodore 64 etc). The other side held the same data in the native Spectrum format; the documentation suggested that this version should be used on the built-in tape deck of the Spectrum +2, which apparently didn't work so well with UniLODE. Accordingly, the program tape had differing versions of the game engine on each side, one using UniLODE and one using native Spectrum tape routines.
