Hello,
The AFG31021 alone will not meet your requirement as neither a two channel model or feature differential output. If you have access to a second AFG31XX1 you can use the Multi-Unit Sync functionality to create the needed waveform. You can find information about Multi-Unit Sync starting on page 50 (PDF page 72) of the user manual.
The second option would be to use an AFG31XX2 model. This would allow you to create a waveform in channel 1, and then set channel 2 as the compliment.
AFG31k User Manual:
https://www.tek.com/en/signal-generator/afg31000-function-generator-manual/afg31000-series-arbitrary-function-generator