Why does a differential amplifier need two transistors?
A differential amplifier generally has two transistors for symmetry, because in many applications, it is desirable for both inputs to have similar characteristics, and for them both to be referenced from the same node (e.g., ground).
You can create a differential amplifier from a single transistor by applying the signals to the base and the emitter, but the input impedance seen by the two sources will be very different — and there's also a DC offset voltage between them. In some applications, this doesn't matter.
In your diagram, you're simply adding voltages by placing them in series, which is not a &quot;differential amplifier&quot; at all. In many applications, both sources must be ground-referenced, so this configuration is not suitable. It also means that the source impedances are all in series, which will cause problems unless they are very small — close to ideal voltage sources.