Each lender has it’s own rules. Call around and ask the max ltv on investment property for a heloc. When I was underwriting consumer (not FNMA) loans, we’d go to 70% on any kind of cash out loan on investment.
All those whose name is one the deed must sign documents to permit a mortgage to be made on the property. I am in the business, and I have never heard of a lender allowing a loan as you described. Most lenders do not do helocs on investment property, especially as tight as the market is now. In addition, the lenders are more cautiously looking at appraisals, expecting that value may go down.
As long as you own under 10 properties it should be easy.
Email me if you have questions.