Superpositioning of fire

When placing the candles next to each other you effectively create a single "fire". We know, from work by Thomas et al (1961) that the flame length is: $$l/D=fleft(frac{dot m^2}{rho^2gD^5betaDelta T}right)$$ Where $l$ is flame length, $D$ is diameter of fuel, $dot m$ is fuel mass lose/flow rate, $rho$ is fuel density, $g$ is acceleration due to gravity, $beta$ is expansion coefficient of air and $Delta T$ is average excess temperature of flame.

Therefore if you increase the mass loss rate the flame length will increase. In your case you will be increasing the fuel mass loss rate as you're increasing the burning area and increasing the heat feedback to the fuel surface and hence the vapourisation of the wax.

Simple empirical relationships have been developed for pool fires for a range of fuels. For example by McCaffrey (1995)$^1$:

$$l/D=0.23Q_c^{2/5}-1.02D$$

Where $Q_c$ is the convective heat release rate.

1: McCaffrey, B., Beyler, C.L. and Heskestad, G., 1995. SFPE handbook of fire protection engineering. Flame Height.” National Fire Protection Association: Quincy, MA.