Double amping is additive. If an amp with high crosstalk is fed a signal, the signal it outputs is going to have high crosstalk. Another amp won't reduce that, it will at best take the signal it's given and add nothing else to it. A clean amp like the O2 only has one job, which is to pass on the signal it's given. It's not supposed to correct it. So if it's given something with high crosstalk, it'll give you high crosstalk.
Likewise, Dolby Headphone effects will remain. But so will any noise and distortion (and crosstalk) from the Mixamp. There's no point to it, unless you need more volume or whatever sort of connectors the O2 provides.