If discs warp its due to heat,so skimmimg them makes them thinner and less able to handle heat.
So buy 2 new reasonably priced quality discs and clamp em on then job sorted or take off warped used discs,find someone that might skim them and refit a now inferior product plus pay to have them skimmed and mess about burning petrol/diesel and using up time while the quest for saving money is not reached.
Now if you have just warped 2 expensive ventilated discs on a cosworth and you are flogging it and want rid of the horrendous brake shudder then get a cheap skim job from your mate and flog it on,thats why brake disc skimmimg was popular in the motor trade.