If you mean taking a HELOC on my primary residence and using that to invest, no. I know lots of folks do that, but my goal is to have my primary paid off. I don't consider that to be an investment in any way. Its a money pit that gives me a place to live. Plus, I don't want to depend on the success of my RE investments to have a place to live.
I did take out a 401k loan some time back to jump start the process. Some people say that's a bad idea. But that's investment money anyway, so I consider this just an alternative investment. I also got very lucky and pulled this chunk of money out before the stock market crash. No great insight there, just luck.
One of our local hard money (squishy money, she call's it) lenders will allow you to cross collateralize one property to buy another. Her normal guideline is 70% of purchase and 70% of rehab, so you have to come up with 30% of both somewhere else. If you have enough equity in another property, you can use that as collateral for the 30%. The LTVs and her lower than typical rates is why she says is "squishy".