I don't get it. It will actually be bad for BOTH chains if indeed Bitcoin forks into two. Some people who are following the space could see this as a weakness and another reason not to start investing or accepting BTC as payments. Imagine if you were a merchant and you have to go thru and be involved in this sh*t, with all the debates and the drama. And worst of all the uncertainty of BTC's future.
Because one of them wants to allow Bitcoin to scale, the other doesn't. duh
I think merchants will support BTC more than BU at least in the beginning because they have been using bitcoin for sometime now and all of a sudden they may not be comfortable in accepting BU. Yes, it can cause some uncertainties which can make the price to correct in the case of BTC. And BU like any other altcoin may get pumped and then dumped, like what happened to Zcash.
It depends many merchants rely on other services to process their sales and are not Point of Sale service providers themselves.
If the main merchant networks decide to accept both forks as valid proof of payment options immediately or add Ethereum, Dash, Litecoin etc onto their list of accepted payment options then it would reduce some uncertainty.
The risk then shifts to the price the merchant sells the product for, the merchant provider already knows that Bitcoin can be volatile unless they have an option to convert their purchases immediately to cash so they may choose instead to keep a portion of the Bitcoin from their sales and accept the coins or sell them immediately resulting in more downward pressure.
In essence the merchant only knows something is up when a customer transacts with a chain the Point of Sale Terminal does not support, otherwise it can be relatively seamless but at the immediate fork only Core would be accepted since merchants would need to update the terminals.