A two-pole system can never be unstable?
Unless there is more to your question, here it goes,


If a system is unstable, with any pole on the open right plane, you will not be able to plot its bode plot (or at least that plot will not have any physical meaning, since for any sinusoidal/DC input the system will grow unbounded).
If a system is unstable, with no poles on the open right plane but some on the imaginary axis, you will be able to plot most of its bode plot, but for some frequencies it will grow unbounded. But in this case, the indication that the system is not stable will come from an infinite peak at  some frequencies.
There are plenty two pole systems that are unstable, such as,
$$ H(s) = \frac{1}{(s-1)(s+1)}, ~~ H(j\omega) = \frac{1}{(-1+j\omega)(1+j\omega)}.$$