Hi there Adam!
Yes it’s generally advised to only use one of the import methods since there could be some errors with duplicate transactions messing up the cost-basis.
If however you’d really like to use a combination of both I’d suggest you use the CSV import for as much of the transactions as possible, and then when you set up API sync you can choose to only import transactions that happened after a specific date.
So imagine you import statements with history from the beginning of your account up until September 20th. What you can do is set up API sync to only pull transactions that occurred after September 20th, that way you avoid having duplicates.
The Binance API unfortunately has some limitations so if you notice a lot of incorrect balances/cost-basis then I’d recommend sticking to CSV uploads.
I hope that helps, but just let me know if you have any follow-up questions!