To record songs in the Sing! App with multiple singers at the same time you need to use a headphone splitter cable. The most simple ones available let you connect two headsets to one headphone socket. They have a TRS (Tip-Ring-Sleve) jack with 3 contacts on one side and two sockets for the headsets on the other side.

Other popular splitters (like this one from Belkin) allow you to connect up to 5 headphones/headsets at the same time. 

Things to keep in mind:

• These splitters only split the audio signal for the headphones. They don’t support the microphone your headsets might have. So your device will usually switch to the internal microphone when you connect the splitter. 
• The more headphones you connect, the more the volume in all headphone will decrease. 
• Make sure the splitter is specifically advertised to split the headphone signals. Other splitters can look identical, but are wired differently and serve a different purpose, e.g. splitting the microphone and a headphone signal. Those splitters won’t help you when you use the Sing! app on your phone. 

From a Smule meetup with Angels_nadine