[NobodyNada]

The PPU (and variants of it) was used in quite a few arcade machines, in addition to the NES.

I don't know if there were any actual machines that used dual PPUs, but the functionality was likely intended for creating an arcade machine with dual-layer background graphics.

[ndiddy]

The RGB PPUs used in arcade machines didn’t have the master/slave functionality, they instead used those pins for the analog RGB output.