You can amend a return as many times as necessary.
You should wait
until the first amendment is processed and accepted by the IRS before sending
in the second one to ensure they are done in the proper order.
You will have to
remove/delete form 1040X from your taxes without removing prior changes made. Then you will start the amendment process again and add the new
1099.
You may track status
of the first amended one using the below link:
https://www.irs.gov/Filing/Individuals/Amended-Returns-(Form-1040-X)/Wheres-My-Amended-Return-1
See this TurboTax support FAQ for
amending a tax return - https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed
