I've been trying to understand how margin requirements work for short positions (assume Reg-T US) to model for a backtesting software (I don't have direct experience with shorting, yet)
I think I understand how this works for 1 stock, you can find plenty of examples (here's one https://www.investopedia.com/ask/answers/05/shortmarginrequirements.asp), but how do multiple shorts interact with each other?
To illustrate, here's a scenario.
Account has $100,000 cash, no long position, and the goal is not to pay margin interest. Ignore fees (commission, borrowing the stock...), and maintenance margin to keep things simple, I think this can be extrapolated later on.
Sell 1,000 shares of ABC for $50 a share, so this requires $50,000 plus 50% initial margin requirement, so the total margin requirement is $75,000. This is under the $100,000 cash, so not paying margin interest. [Edit: based on @bob-baerker answer to this question, I'm not sure this is even correct, so feel free to correct me here]
So now, how much of XYZ, also at $50 a share can I short without paying interest?
Does the total margin requirement need to be $25,000 or less, or can the $50,000 you get from selling ABC be used to secure the margin, short more, and still not pay interest? Can anyone shed some light on how this would work.