Yes, you can file a second amended tax return.
You should wait until the first amendment is processed and accepted by the IRS before sending in the second one, just to make sure they are done in the proper order. Use the IRS's Where's My Amended Return? site. to track the first amendment.
To complete the second amendment you will need to manipulate your return by deleting the Form 1040X, which will leave you with a 1040 "original" return that includes the changes from the first amended return. Then, you will start the amend process over again to put in the information needed for the second amendment. When you start the second amendment the balance or refund due should be $0.
