Take two very knackered, very decrepit rusty old Claude Butler mountain bikes in horrid lime green.
Strip them both down to their component parts, pick the best from each and clean and service all parts.
Take the best frame, sand it back to the primer or where's there's rust back to the bare metal. Re-prime then spray with a special black undercoat ready for the pearlescent Blue top coat.
Re-fit forks, bottom bracket, cranks, main gear, rear gear, handle bars, saddle... then re fit brakes, brake levers and shifters.
Pop to the local bike shop and buy a length of brake cable and a long strip of outer then refit and tune it all and voila! one perfectly good mountain bike rescued from two piles of rusty rubbish not even worth the petrol to take them to the tip!
This is what one of the bikes looked like... uncared for and unloved for years...
And the other one, now fully restored to working order and with a spiffy new coat of paint...