A great feature for users would be to have a sandbox or mirrored account. Uploading new wallets or spreadsheet sometimes goes very wrong and messes up live Koinly accounts. Having a sandbox that could tested on, then allows the ability to merge the sandbox into a live account once issues are resolved would really help.
Nice suggestion - most of our users create secondary “free” accounts to play around with the data.
I understand what you are proposing, but this would require a massive amount of server capacity if you have 2 sets of accounts for each user etc. Calculating times would also suffer. 
Feel free to post this idea on our feedback board, if it gets enough votes, then I am sure the product team will give it some thought. 