I have always recommended dual subwoofers at a minimum to all my friends setting up HTs regardless of room size.
The benefits in terms of increased headroom, lower distortion and smoother frequency response are very real.
While two of the same subwoofers are ideal, its not necessary in order to get most of these benefits. If going with different subwoofers I would suggest trying to use 2 subs that have close tuning frequencies ( also called Fb or box resonant frequency). It will make it easier to get the phase right between each sub and better blending with the speakers, and result in smoother FR. Its a phallacy that you have to use the same manufacturers subs to get these benefits and any two subs with similar Fb should be OK if their other design parameters fall in line. Even if that's not possible to have similar subs, dual subs are still usually worthwhile because of the extra headroom and lower distortion.
Fb of the EP500/600 is around 22/23hz and I suspect given the similar dimensions of the EP350 it would also be in that vicinity so they would pair really well. One thing to keep in mind is there will be a slight time delay because of the DSP processing in the 500 so if you can, position the 500 one metre closer to the listening position than the 350 for optimal placement.
