I would think long and hard about if you actually need to carry a second spare.
I drove 40,000 mi. from Alaska to Argentina, and while I had plenty of flats, I never even put the one spare on the Jeep. Every flat was either slow enough to just get it patched in the next town, or the one that was kind of bad, I just filled it with my 12v compressor and was in another town in 20 mins getting it fixed. (It was pouring rain, and I had a bad stomache bug, so I couldn't be bothered changing it out myself.. yeah, that was a lazy day)
Many round the world overlanders I met had never used their one spare, let alone their second.
Almost all of them wish they never brought a second to save the weight.
One guy had traded his years ago at a border in Africa, and never regretted it.
-Dan