No, it would not be correct to say that two objects moving in opposite directions at half the speed of light are collectively moving at the speed of light relative to each other.
According to Einstein's theory of special relativity, the principle of the constancy of the speed of light in a vacuum states that the speed of light is always the same (approximately 299,792,458 meters per second) for all observers, regardless of their relative motion. This means that the speed of light is an absolute limit that cannot be exceeded by any object with mass.
In the scenario you described, both objects are moving at half the speed of light relative to an external observer, but their relative speed to each other would be less than the speed of light. This is because velocities do not simply add up in a linear manner in special relativity.
When velocities are very low compared to the speed of light, the classical laws of addition and subtraction of velocities approximately hold. However, as velocities approach the speed of light, relativistic effects become significant, and the addition or subtraction of velocities requires the use of special relativistic equations.
Therefore, in the case you presented, the relative speed between the two objects would be less than the speed of light, not equal to it.