There is indeed a different volume bar for when you are on a call. I think the easiest way to set this is to change the volume whilst on a call, this will save the settings for the next time you're on a call. If you want to change the settings before you go on a call just phone someone yourself and change the settings, they don't even have to pick up for you ...
Go to contacts -> select the contact which you want to change the preference -> deep press on the number -> select clear calling sim.
Now you will be able to make calls normally with the sim card you wanted.
So I've discovered there is a way of doing this.
I found this answer on Quora:
There's a video there and the answer is essentially :
Open the phone app Press the Menu button (3 dots top right)
Press Calling Accounts
Press on your call Operator (Was EE in my case)
while browsing on the internet on sim card 1, can I get calls to sim card 2 and also call to somebody else?
You can call someone/receive calls in SIM 2, but you won't be able to use the internet from SIM 1 during the call. But if you call someone/receive a call on SIM 1, you can still use the internet from SIM 1 during the call.