The Triumph motorbikes were stolen from Firs Drive between 10pm and midnight on August 19.
It is believed the suspects rode the motorbike out of Harrogate and headed towards Leeds.
The motorcycles had the number plates AV52KWS and WX56EKY, but these are likely to have been changed since.
Police have released a picture of the 56-plate bike.
The front right fairing on the other bike is damaged and has a messy fiberglass weld running through it, with a crack on the rear left seat fairing.
North Yorkshire Police are appealing for information from the public.
They would like to speak to anyone who saw a Ford Focus in the area at the time of the theft, or any sightings or adverts of the bikes for sale.
The owner of the bikes is offering a substantial cash reward for information given to the police which results in the safe return of both bikes.
Anyone with information that could assist the investigation should contact North Yorkshire Police on 101, select option 2, and ask for James Skaith. You can also email [email protected]
If you wish to remain anonymous, you can pass information to Crimestoppers on 0800 555 111.
Please quote the North Yorkshire Police reference number 12200144624.