can I share files between iMac on Catalina and macbook pro on Big Sur?

Depends on what exactly you mean by "Sharing files".

How are you sharing them exactly?

There should be no issues moving files using a USB drive, or over Airdrop or the network or even by email no. Whether an App running on Catalina and an App running Big Sur can open the same file will depend more on the app not the operating system as much.

In general there should be no issues, bit until people can test a large number of applications on Big Sur there's no way to be 100% sure.

Due to terms of Apple's Big Sur beta, and of these Apple Support Communites, we cannot discuss Big Sur at this time. When it is released, there will be a dedicated Big Sur community here for these questions.

Don't see any issue sharing over iCloud Drive if that is what you mean.