<shrug> i've just been installing to c:\K-meleon, which works fine, but i haven't tried having two installations at once... maybe there's some conflict when the installer tries to write to the registry? you could try removing any existing k-melon registry entries first, since i don't think k-meleon really keeps anything very important there. otherwise, maybe someone has some experience having two installations, and can help... it may help to know what platform you're using? </shrug>
maybe i'll try installing the next beta alongside the current one, for fun...