It can be done using bitwise operators '<<' and '|'
public int Combine(byte b1, byte b2)
{
int combined = b1 << 8 | b2;
return combined;
}
Usage example:
[Test]
public void Test()
{
byte b1 = 0x5a;
byte b2 = 0x25;
var combine = Combine(b1, b2);
Assert.That(combine, Is.EqualTo(0x5a25));
}