How is the energy conservation principle preserved in the case mentioned below? Suppose there are 2 sinusoidal signals $\cos(\omega t)$ and $\cos(\omega t)$, which implies that their individual power is $1/2$ unit and total power is $1$ unit. When added, the signal $2\cos(\omega t)$, however, has power $2$ unit.

Obviously, 

$$\rm Energy_{signal} = Power_{signal}\times Time_{period}$$

So, how does the principle of energy conservation hold in this case?
Power is homogeneous to a second degree quantity. When the data is doubled, the power is quadrupled.

The energy here (classicaly) is defined in an Hilbert space as a squared $L_2$ norm, deriving from some inner product $\langle\cdot,\cdot\rangle$, assuming a real Hilbert space,  $\|x\|^2 = \langle x,x\rangle$. Hence:  $$\|x+y\|^2 = \langle x+y,x+y\rangle = \|x\|^2 + 2\langle x,y\rangle+  \|y\|^2 \,.$$

Now set $y=x$, you gracefully get $\|x+x\|^2 = 4\|x\|^2$.