lets break it down
car set to future travel+tardis set to future travel=going to the future of first activated items future path. the car is set to a time in the future but that does not mean it will travel to that time regardless, it only accelerates the car to near the speed of light for a long enough duration to get to target time from original time period of which it was set. so if you hit a tardis set to a future time with the car set to a time 5 years into the future, you will go to the time of the tardis plus 5 years and possibly in a different variant of the future if branch multiverse theory is correct.
let z=resulting time,x=delorean, and y=tardis
5+y=Z
in the instance where both go toward opposite directions in time, it will result in either cancelling out or going foward/back in a certain value. for example(let the "-" mean backwards time travel:
-6"x"+8"y"=z=2 years in the future and vice versa
but of course there is a problem on who would be first in an opposite time travel scenario assuming both were activated and would take effect at the same time. it will still work and here is how.
to go back in time you have to go faster than light, and to go foward in time you just go at the speed of light.
assuming both are the same mass, the momentum of the item going back in time would be larger even if both are at infinity (since any mass at the speed of light would technically be infinite). because despite what most think, science says infinity+1>infinity.
so if both were to strike and time travel at opposite ends of the timeline equally that would mean that the result (even though the target times are different) means you will go back in time first before moving foward.
now as for both going back at the same time with the same mass and same travel execution time. that means both will be at the mass of infinity+ at faster than the speed of light.
this has me stumped as i'm typing so...hold on...
dammit....how...
i could mention quantom mechanics but that's just a cheap excuse and an easy way out for sci fi movies...
hmm...
well, one theory of mine is that the delorean would time travel backwards first since its already accelerated to around 88mph (time traveling gadgets speed to operate time warp) that means it would have a 1up on time travel priorty.
i'd like to have some opinions on this from you guys, this just some guess work on my end.