*'m used to the Linux install asking me which drive to use and the Zorin install is a little confusing, I'm afraid that I'll mess up the other distro. Would be nice if it asked if I wanted to just use 1 of the drives. *
If it were me, I would physically take out the drive you don't want to use for Zorin, and only leave the drive physically connected you want to use for Zorin. Do the install of Zorin, make sure everything works, then physically re-connect the other drive.
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You could do it like @bu11etpr00f already wrote with unplug the Drive that you don't want use.
Or You choose the Installation Option ''Something else''. There You can choose the Drive You want use and create the Partitions.