I've got about $525K in equity in the property, but lenders have told me that I cannot get a HELOC to access that equity for additional purchases because the duplex was financed as an investment - they have said TX law dictates that HELOC can only be used on primary residences.

Quote from @Eric Huntermark: Additionally, as I'm thinking about conventional loan products as well.

I should have ran the plumbing for it even if I didn't plan to add it.  

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Part of that is adding additional cost for increase in building products. 

For this I’ve taken the payment amount of the second loan and included it as an additional monthly expense.

They charge $1250 for properties under $1mm for a 1 to 1 exchange and if you have another property to move your funds into, they charge $550/additional property.

Those are some of the unique things we offer in addition to helping each investor build a strategic investment plan & roadmap to achieve your specific goals.

It is much better to have a reserve fund and add it monthly.

What would be my best route to take if we need additional finances?