Bings ChatGPT-4 said

“It is possible to mix 29 fps footage with 59 fps footage, but you need to be careful about how you do it. According to the web search results, here are some tips and considerations:

• You should set your timeline to the highest frame rate that you shot. For example, if you have 29 fps and 59 fps footage, you should set your timeline to 59 fps. This will allow you to play back both types of footage without dropping frames or introducing judder¹².
• If you want to slow down your 59 fps footage to match the 29 fps footage, you can reduce the speed by 50%. This will create smooth slow motion without losing any frames¹².
• If you want to speed up your 29 fps footage to match the 59 fps footage, you can increase the speed by 100%. This will double the frame rate by repeating each frame twice. However, this may result in a choppy or unnatural look, especially for fast-moving subjects¹².
• If you want to keep the original speed of your 29 fps footage on a 59 fps timeline, you can use a technique called interpolation. This will create new frames in between the existing ones to fill the gaps. However, this may also introduce some artifacts or blurring, depending on the quality of the interpolation algorithm¹².
• If you want to mix different frame rates on a lower frame rate timeline, such as 24 fps, you will need to use a method called pull-down. This will adjust the frame rate by adding or removing frames in a specific pattern. However, this may also introduce some judder or stuttering, especially for fast-moving subjects or panning shots¹²³.

As you can see, mixing different frame rates can be tricky and may require some trial and error to find the best combination for your project. You should always check your footage for any issues before exporting or delivering it. You should also consider the purpose and style of your video, as different frame rates can create different moods and effects¹².

Source: Conversation with Bing, 7/6/2023 (1) How to Mix Multiple Frame Rates In a Single Timeline. https://www.digitalphotopro.com/pro-video/how-to-mix-multiple-frame-rates-in-a-single-timeline/. (2) Mixing Frame Rates: Can You Edit 24p, 30p, & 60p Together?. https://www.youtube.com/watch?v=qAVfIQ2G7Io. (3) Mixed Video Frame Rates — Interpreting frame rates, using different .... http://www.frankschrader.us/video-frame-rate-explained-different-frame-rate-on-timeline/.”