I love a duo, but they can be hit or miss.
Some are great when the girls are enthusiastic and relaxed. I often also like to make it so that one person is getting the attention. It can be great fun teaming up with a girl and fucking another girl. She's sucking her clit or tits while you fuck her, or grinding on her face etc.
Really objectify the girl who's getting fucked and talk to her with the other girl who you are fucking her with, love it.
Another way I like is just to get all the pleasure myself, have my balls sucked while I fuck or get sucked by the other girl, and stuff along those lines.
Downsides are when girls get prudish, spend half the time getting new rubbers out because some saliva or pussy juice from the other girl might be on it.
Also it's quite common for the girl who hasn't got a cock in her to get lazy, and just do fuck all, or just some pointless shit like rubbing your back.
But when it goes well it's the most mindblowing cum exploding fuckest that you'll get addicted to!