Why is the axial tilt of a planet expressed as a single angle when two angles are necessary to specify a point on a sphere? Imagine if the Earth’s axial tilt were 90 degrees, then the axis of rotation could point anywhere along the circle in the orbital plane. In the extreme cases, either always point directly towards the sun or always point directly towards the direction of orbit. In the former case, half of the planet would have eternal day or night, and in the latter, each half would have equal length day and night. A second angle could distinguish the two.
The Earth's axial tilt is expressed as a single number because it's just an angle. It can be considered to be the angle between the plane of the equator and the plane of Earth's orbit, or the the angle between its axis of rotation and its orbital axis. Its axis of rotation always points to the same two locations in the celestial sphere where all locations are expressed as two angles. The north location is near Polaris.
By "always" I mean right now, over a time scale of a few years. The point actually moves around in a circle over the course of 26000 years. There are other, smaller deviations in a much shorter time scale. Search terms "precession" and "nutation" will find explanations with animations. Neither phenomenon relates to your question, though. The amount of tilt over periods of millions of years stays between 22 and 24.5 degrees, even though Earth's extended axis moves fifty some degrees in thirteen thousand years.
Technically, the Earth’s axial tilt (or that of any other planet, for that matter) is expressed as two angles, namely the right ascension and declination of the point on the celestial sphere towards which the north pole of that planet points. For example, in Earth’s case, it’s 0 h and +90°. For Mars, it’s 317.68143° (21 h 10 min 44 s) and +52.88650°. And so on.
Axial tilt, also known as obliquity, is the angle between an object's rotational axis and its orbital axis, or, equivalently, the angle between its equatorial plane and orbital plane.
To distinguish between one or two angles application: There is the angle of inclination which is measured between a reference plane e.g. Invariable plane and the plane of the orbit. Then there is the Longitude of the ascending node which determines the direction that the inclination is pointing. Which is all about orbital rotation and not about the questioners planetary rotation.