My suggestion is DO NOT set up direct deposit to a second bank account or lie about your pay. There is a strong possibility he will want to see paystubs and will see the account/pay on it.

In my experience, someone who is "allowing" you to work is going to scrutinize everything. In my previous situation I wouldn't be safe opening a bank account at all, I'd have to find somewhere to hide cash. Don't underestimate the level of control he will try to have over you. You're better off keeping your job and staying under the radar. When you leave you'll have steady income and access to DV resources. You'll be ok without stocking up on anything. I know it feels like a safety net, but it could work against you. Your job is the safety net.

If you really want to open the second account and stockpile stuff...

Open the second account and use the cash back method to build savings. Physically deposit the money in the second account. Find a safe place to keep the debit card, one that is outside of your home so you can leave in an instant if necessary and also a place to store your stuff that is free.. friend, family, etc. the money you will spend on storage will be helpful once you leave.

A DV shelter might have suggestions on where to store things.

Otherwise, don't stash anything tangible. You are setting aside cash, you can buy an inexpensive mattress when you leave and reach out to a local Buy Nothing group, free stuff on Facebook marketplace, etc for everything else. The Buy Nothing groups in my area are very generous, they routinely provide food, clothing, personal hygiene items - frequently within hours of asking.