It depends on the power output of your DVD player as well as cable & connector attenuation characteristics. I usually use .5 of a Db loss for each connector as a standard measurement. The rest is just math.

With respect to the difference of cable length the input device spec of the display should be able to clarify that question (i.e. + or - %latency).

So, without having to do all of the research and math wouldn't it be easier to just buy a second DVD player as previously mentioned by willywaxer.