I would have some concerns about this strategy.
Perhaps your experience is different, but the landlords I know who've done well and still enjoy being a landlords typically tried to pay of one property at a time. Then, they were able to use 100% of their net cash flow from the paid-off properties (since they didn't have a mortgage anymore) to purchase their next property.
My main concern for you would be that you would be adding debt without adding any additional equity. I would also suggest that you won't actually be increasing your net cash flow by much, since most of the new revenue from the second property will be going to pay the additional HELOC payment as well as the second mortgage on the new property, since you are only paying a down payment. You would be in control of more property, but not much more profit. In fact, if you have trouble with the property values, or getting tenants who pay on time, or vacancy, you could have additional problems.
I would also be concerned that if anything happens to your main source of income, you'd be at greater risk since you would now be carrying more debt. If I were you, I'd focus on paying down the first mortgage so that more free cash flow becomes available for additional properties. If I couldn't put together a proper down payment from cash rather than additional debt, that would be a sign to me that it may not be the right time to take on more debt.
I know this isn't the conventional wisdom, but just my two cents.