is it possible to hardware mod a router to have two spi flash chips , with different chip selects and mount them as one chip ? lets say two 4MB chips, and openwrt sees them as 8 MB ? or maybe sees the second chip as an mtd partition?
note: I'm fully capable of any hard / soft mod required.
Not really, SPI means serial peripheral interface.
The select and clock are parallel but the io is serial
Sounds like you are looking for "mtd-concat". This feature is available for openwrt 4.14 and 4.19 and currently, some ath79 buffalo devices are already using it.