Why does my digital upconversion result in two tones?
You can use the property of Fourier transforms which says that: $x(t)e^{j2\pi f_0t}\leftrightarrow X(f-f_0)$. That is, multiplying the time domain signal with a complex exponential will give you a shift in the frequency domain. 

By multiplying by another sine, you get: $x(t)\text{sin}(2\pi f_0 t) \leftrightarrow X(f)*\text{FT}\big(\text{sin}(2\pi f_0 t)\big)=\frac{X(f-f_0)-X(f+f_0)}{2j}$, and so you see an extra peak at $120$ MHz.