I have a substantial amount of margin interest that is secured by stock and its dividends. The money was used to buy a 2nd home. Is the interest deductible?

No, it is not deductible as mortgage interest. Your loan is not secured by the home, it is secured by the stocks in your brokerage account.  A margin loan does not qualify as a Secured Debt for purposes of the mortgage interest deduction. According to the IRS:

You can deduct your home mortgage interest only if your mortgage is a secured debt. A secured debt is one in which you sign an instrument (such as a mortgage, deed of trust, or land contract) that:

In other words, your mortgage is a secured debt if you put your home up as collateral to protect the interests of the lender. If you cannot pay the debt, your home can then serve as payment to the lender to satisfy (pay) the debt. In this publication, mortgage will refer to secured debt.

As you describe the transaction, you wouldn't be able to deduct this interest.  

The money you borrowed was secured by stock holdings, not by the real estate you purchased. In order to deduct as mortgage interest, the "loan" would have had to be a mortgage , i.e.secured by the condo.

You described the condo as a "2nd home", i.e. a personal residence. If you borrow money to finance investments, the interest you pay is considered investment interest. Examples include margin interest your broker charges you on loans to buy stocks, and interest you pay on money you borrowed to buy raw land for speculation. If you have investment interest expense, you can deduct it up to the amount of your net investment income. A "2nd home" does not qualify as an investment (we all know it might turn out that way, but on its face it is not).

Do I qualify for the investment interest expense deduction?

Eligibility requirements

 - You must be an investor who borrows money to buy investments, and receives interest, dividends, capital gains, royalties, or other investment income.

 - You must itemize your deductions on Schedule A.