I am unaware of any doubled up flipper ROMS for that purpose. Sounds like they just take a 2K image and concatenate two copies of it to make it a 4K.
A system 7 board can run system 6 roms, but the jumpers need to be changed on the MPU board so it behaves like a system 6. When it does this, the 2532 (4K chip) can be replaced by a 2716 (2k chip) so you wont need any doubled up images.
You can find the time warp images here: http://www.planetarypinball.com/mm5/Williams/tech/roms.html