How can I reinstall serum for another OS

I have a serum for win10 and I am using it within FL Studio 20
I purchased it through www.PluginBoutique.com
Now I’m going to change my win OS to mac OS

there are my purchased Product number of serum and download link in Plugin boutique
but they say ‘That product key has been used already. ‘
How can I download it and reinstall?

If I want to install Serum both on Mac and win on same computer (multibooting) , So can they both run on shifts?

You are already registered so you don't need the Plugin Boutique voucher any longer. You will find both installers in "your account" on the website. Running dual-boot won't be any problem.

Warm regards,
Steve