1. Make sure at least two controllers are on and connected to your xbox one (if you don't know how to do this, there's a tiny button on top of the controller and a small button that's sort of hidden somewhere on the xbox itself. Press both buttons and the controller should connect to the console.)
2. Sign in with both controllers by pressing the A button on both controllers when it shows the option to "Press A" in the upper-right corner of the screen. A white menu should pop up on the screen giving both controllers the option to select a profile.
3. In order to play splitscreen online you must do this. Player 1 must sign in as a profile that has an xbox live gold membership. To help you remember, a profile that has an xbox live account should say "Xbox Live" under the gamer-tag or profile name. Remember that it will say "Xbox Live" as long as you're connected to xbox live with that profile, either with a gold account or a free silver account. If you have the free silver account, then you will not be able to play in online matchmaking.
4. Player 2 will need to sign in as either another profile with an xbox live gold account or a guest. The guest option has a blank white gamer picture and is marked as "Guest." When a guest is signed in, it will show the gamer tag of the guest as Player 1's gamer tag with an added "(1)." When playing splitscreen with up to four players and all 3 players after player 1 are guests, then it will look something like this.
Player 1: OriginalName
Player 2: OriginalName(1)
Player 3: OriginalName(2)
Player 4: OriginalName(3)
In the case above, Player 2 is the first assigned guest, Player 3 is the second guest, and Player 4 is the third guest. Note that guests do not have the option to customize armor pieces, firefight voices, and amor effects and ranking up is disabled. Guests can however change the color of armor and emblems. Guests will be stuck equipped with the default "Mark V (B)" armor set.
I hope this helps.