What's the best way to open a separate A/C for short term trading?

Hi there

I already have an account with CommSec and I'm a long term investor.
Now I want to do short-term trading, but for tax purposes I don't want to mix my trades with my long-term holdings.
I wanted to open another account for short-term trading, but CommSec told me that's not possible. But I understand a lot of people do operate separate accounts.

How do people do it?