When flying, do I need to use the rudder and ailerons simultaneously? or will I be able to execute a good turn with only the ailerons? Should I use a Y-harness to connect both the ailerons and rudder on the same channel to make it easier?

Most all RC airplanes equipped with both ailerons and rudder will use the ailerons as primary function for turns.

Of course if it is a three channel model airplane, most of time rudder will be on plane and not ailerons along with elevator and motor control. In this case rudder is the control for turns, but some builders will select ailerons for model and not have rudder, then ailerons are used for turning.
Rudder is an effective way to turn if plane is designed with this in mind.

Ailerons will generaly produce a better looking turn because the roll is cleaner. Rudder will produce a barrel roll.

Some four channel scale planes will do better turns with co-ordinated use of both rudder and ailerons in the turn. A scale model of a J-3 Cub comes to mind as a prime example of this. They usually need to be flown just like the full scale one with use of both rudder and ailerons. In a case like this, you could use a Y-Harness to couple both servos or use a mix function in transmitter to do same thing.

Bottom line:
Most of time, RC model airplanes use just ailerons and elevator for turns.
Some few exceptions where mixed ailerons & rudder along with elevator produce better results.