No room underneath like an 80 with a windy-uppy thing? I don't know if you have extra tank under there.
You could find a thinner tyre with same rolling diameter so that you have one full sized off road spare and one sort of space saver get you going spare.
Just giving you options. I guess some of this depends where you are going and what size you have on. If you were going round the UK with 265.70.16s or something you might get away without a spare at all. But across the sahara with something weird and wonderful then you might need three spares!
Just trying to understand the need for two spares on the vehicle. Given that you can carry repair outfits, tyre goo, compressors etc.
Could you get a single carrier and then have it modified to take second spare? Hole, spigot, arm, bzzzz with welder? Could you just carry the tyre and no wheel. Changing a tyre with levers is not hard.
Chris