Yes you can. A mortgage is simply a lien against a piece of property. Think of it this way, you are pledging the property as collateral for repayment of the loan. The bank, by recording a mortgage against the property is saying that if you were not to pay the loan back, they would have the ability to start foreclosure, and repossess the collateral to try to gain their money back.

More to the point of your question, there are all sorts of lenders, with many different specialties. Some lenders specialize in first mortgages, some in second mortgages, others in both. There is no rule that, if you have a first mortgage, all other mortgages on that property must be with the same lender. As long as you qualify for the second mortgage, and there is sufficient equity in the property, any lender who writes second mortgage loans will fund your loan, and file a mortgage on the property behind your existing first mortgage lender. You can actually, assuming it fits guidelines and you have the equity, obtain additional financing beyond a second mortgage. The key in these cases are your qualification, and the value of the property, not so much who the lenders are that have given you the loans.