Booking a R/T for 2 - with different return dates - Same PNR?

I am planning an international trip for myself and my niece -- we are traveling together outbound, but returning on different dates. I'd like to have us on the same PNR for U/G purposes on the outbound leg --

Should I call the 1K line to book this? I have no idea how I would do it on the .bomb.

Thanks in advance for the help!