Can a DFA have two symbols on one arrow?
The transition graph (as a drawing) is merely a representation of an Automaton, which is a well-defined model.

Formally, an DFA is a tuple $(Q,\Sigma,\delta,q_0,F)$, where the "type" of the transition function is $\delta:Q\times \Sigma\to Q$.

Thus, if you have $\delta(q_0,a)=\delta(q_0,b)=q_1$, that's fine.
In the graphic representation, you will either have two arrows from $q_0$ to $q_1$, labeled $a$ and $b$, or you can just put both letters on the same arrow, it's not a formal thing anyway.