The first appeal comes after a crash on the A82, near Fort William, where a white Triumph Tiger motorcycle and a grey Audi S3 car were involved in a collision.
The incident happened around 3pm on Saturday at Achintore Road.
A 56-year-old man, the motorcyclist, was taken to Belford Hospital but succumbed to his injuries.
The road was closed for more than six hours.
Police Sergeant Alasdair Mackay, of the Road Policing Unit, said: "Our thoughts are with the family and friends of the man who lost his life in this serious crash.
"Enquiries are continuing to establish the full circumstances, and I would urge anyone with dashcam footage of the incident, or any drivers who may have witnessed what happened, to please get in touch with us as soon as possible.”
The second witness appeal follows a crash on the B778, near Stewarton.
The incident happened around 11.10am on Saturday on the B778 near to North Kilbride Lodge, and involved a blue Yamaha motorcycle and a tractor.
A 32-year-old man, the motorcyclist, died at the scene.
Police Sergeant Kevin Blackley of the Road Policing Unit in Ayrshire said: “Our enquiries are continuing into the full circumstances surrounding this collision and our thoughts are with the family and friends of the man who died.”
Anyone with information should call Police Scotland on 101.
A message from the Editor:
Thank you for reading this article. We're more reliant on your support than ever as the shift in consumer habits brought about by Coronavirus impacts our advertisers.
If you haven't already, please consider supporting our trusted, fact-checked journalism by taking out a digital subscription.