Thrust is the wrong measurement to use for this comparison, as is thrust to weight. What matters is the Specific Impluse $I_{\text{sp}}$, which is a measure of the ability to change momentum per unit of propellent.
The RL10C has a specific impulse of 450s, while the Dawn engine is over 3,000, in other words, the Dawn engine can do over over 6 times more work per unit of propellant though its lower thrust means it will take longer to do it, but unless you are trying to escape a gravity well, there is no hurry. One source of the difference is the fact that a chemical motor includes its power source in the mass of its propellant through oxidation, while for an ion motor the power comes from an fission - reactor or solar panels etc. Now the weight of the engine itself starts to make a big difference. To use Jacks example, the 200Kg RL10C with 799Kg of fuel and 1 kilo of payload would produce a $\Delta v$ of:
$\displaystyle \Delta v=v_{\text{e}}\ln {\frac {m_{0}}{m_{f}}}$
Where
${\displaystyle v_{\text{e}}=I_{\text{sp}}\cdot g_{0}}$
$\displaystyle \Delta v=450 \times 9.8 \times \ln {\frac {(200+799+1)}{(200+1)}}$
$ = 7075ms^{-1}$
The 8.3Kg Dawn engine and 2.5Kg of propellant with the same payload would get you
$\displaystyle \Delta v=3000 \times 9.8\ln {\frac {(8.3+2.5+1)}{(8.3+1)}}$
$ = 6999ms^{-1}$
but would be much cheaper to get 11.8 kilos to LEO so that you can accelerate a 1kg payload to 7,000 m/s then getting 1000kgs to LEO to accelerate the same payload to the same speed.
Adding more engines does not change the Specific Impluse, it just increases the fuel flow (ie it increases the thrust), but since you are now moving the weight of the additional engines, it reduces your final $\Delta v$, as rocket scientists love to say, you get there faster but not as fast, (you reach a lower velocity but you reach it sooner). Engines which produce thrust in excess of their own mass can be combined to use that excess thrust to accelerate out of a gravity well, however, ion engines do not have any excess thrust so combining them has limited benefits.
Doing this with two chemical engines would give:
$\displaystyle \Delta v=450 \times 9.8 \times \ln {\frac {(200+200+799+1)}{(200+200+1)}}$
$ = 4833ms^{-1}$
Two ion engines would give you:
$\displaystyle \Delta v=3000 \times 9.8\ln {\frac {(8.3+8.3+2.5+1)}{(8.3+8.3+1)}}$
$ = 3904ms^{-1}$