You are incorrect to say that the value of a Bitcoin fork is the value of the combined value of the bitcoin of the participants of the fork. If the market does not give any value to this bitcoin then it will not have any value, and the bitcoin on the unforked network will still retain it's value.
Why are you assuming the new users that follow the new blockchain/fork aren't considered a "market" into themselves that can trade value , services and goods. This market may revert more to a speculative market as many vendors could drop off but it would still be a market which attributes value to these tokens. Additionally, there are possible case examples where a majority of the merchants, vendors and users leave the asic farms behind and make one blockchain heavily secured but worthless (thus rapidly becoming unsustainable) and the other blockchain being initially less secure but heavily valued and this rapidly regrowing security.
They would be considered to be a market amongst themselves, however it would be much smaller then the "real" market for bitcoin and what is traded in exchange for bitcoin. The value of each additional participant in a marketplace is exponentially more valuable to the marketplace then the last additional participant; thus when a very large percentage of the "market" is using the "other" version of Bitcoin the total value of the market will be much less (in terms of proportion) then the other bitcoin market. This will cause participants to go "back" to the "real" bitcoin which will cause the other market to continue to decline in value until it goes to (near) zero