Most fixers are generally quite acidic whereas developers need an alkaline environment to operate. Reducing the pH reduces development activity, possibly to the point where there is none at all. Secondly, the fixer can oxidise the developer and directly destroy its active components. Thirdly, the fixer will be stripping your latent image off the film at the same time as the developer is trying to develop it, which further reduces the image that you get.
Originally Posted by OEyers
With about 8% of your developer being fixer, the developer may have some activity left in it, but I'm pretty sure it will be quite weak and certainly not to specification - even if the dev still worked at all, you wouldn't know how long to develop for. You could test with a short piece of scrap film with some meaningless test images but certainly don't use it on anything you care about.