Source: Wikipedia, "Sonic the Hedgehog 4: Episode II", available under the CC-BY-SA License.
"Sonic the Hedgehog 4: Episode II plays similar to the previous episode, but with several enhancements. Sonic is now accompanied by Tails, either controlled by the computer or a second player via local or online co-operative play. Unlike in the 16-bit titles, Tails cannot be played in single player and likewise Sonic can't be played on his own. There are also new various combination moves that can be performed together, such as Tails lifting Sonic up to places he would be unable to reach otherwise, and the two can roll into a ball to perform a fast and powerful spin attack.
Special Stages in Episode II, also accessed the same way they are in Episode I, are based on the half-pipe style format introduced in Sonic the Hedgehog 2, in which the player is required to collect a certain number of rings before the goal in order to obtain a Chaos Emerald. Also returning are the Red Star Rings that have previously appeared in Sonic Colors and Sonic Generations. One Red Ring is hidden per act, and an achievement is unlocked after collecting them all. There is also cross-compatibility planned for the XBLA and Windows Phone versions, allowing players to alternate their games between them using Cloud saving.
Owners of both Episode I and II (provided they are both on the same system) will unlock free Episode Metal content in Episode II. Episode Metal explains how Metal Sonic came back to life after being defeated in Sonic CD. Players play as Metal Sonic through four acts, all of which are reworked zones from Episode I, explaining the significance of those zones to the story as well."