So, what would happen, in a market perspective, is that both chains would start with the same value, but the one forked by Core (technically, Core would have to rollback, to reverse the stealing of Segwit outputs
Why would you assume they start with the same value? Did Bitcoin Cash start with the same value? How about Bitcoin Gold, Bitcoin Diamond, etc?
This situation would be entirely different from the other forks. There was no theft when bitcoin cash, gold, etc, were forked. In the case of theft, it would be similar to what happened to ethereum.
When the DAO hack happened, ethereum forked from the main chain, to reverse the hack, and the main chain (at that point) became "ethereum classic". Eventually ethereum became the main chain, gaining more value than the classic version.
Easy to predict such gains in un-regulated / dark / ano / anarcho Environments , isn't it ?