Don't take out a HELOC on this property right now. 

Both properties have a substantial amount of equity that’s essentially sitting idle.Would it be a good idea to tap into this equity (through a HELOC, cash-out refinance, etc.) to fund future fix-and-flip projects or purchase additional rental properties?

Quote from @Matthew Beninate: Quote from @Sarita Scherpereel: Quote from @Seo Hui Han: You may want to consider local banks / credit unions just because many of them will require or give you more favorable terms for things like HELOC's and other line of credit products if you have a deposit account with them. 

Cash out refi only if the rental income significantly exceeds the monthly payment and you want liquidity to invest elsewhere.2.You can maybe do a HELOC if you want a smaller loan that doesn’t involve refinancing the entire house.3.

I've been searching for threads/experience purchasing via DSCR (using HELOC as down payment + rehab) and then cash out-refi at 6mo & pay off HELOC...am I missing something?  

What is the interest rate on the HELOC?

@Tyler KeslingThe Heloc into the DSCR makes perfect sense.

What was the need for a HELOC?

I would first check to see if HELOC is even an option - in TX its hard to find and can only be done for a primary residence.

Get a HELOC, which is possible but more difficult to find on a rental property, and is much easier and cheaper for an owner occupied property.