Comingling doesn't imply gambling.

I could borrow $10 from Jim, and $20 from Sue, and deposit them both into the same Chase account which now has $30. As long as the balance stays above $30, everything is fine.

You could let the balance fall below $30 in your example because it’s a loan (“borrow”), not a deposit.

If you’re claiming to hold the funds on their behalf and do otherwise (like “investing” them in your less than arm’s length hedge fund), then you’re breaking your fiduciary obligations.

Right, the problem is when leverage and borrowing comes in