IIRC from earlier discussions there are limits to the Pads ability to handle higher thrust vehicles than Ares V. Remember that they were designed for Saturn V and even Apollo didn't gratuitously overdesign stuff for the sake of it.
I believe that Ares V will have to be 'clocked' round at an angle to ensure the thrust from the two boosters goes to different sides of the flame deflector as it is. Any more thrust and major modifications to the Pad, such as opening out additional flame ducts to the sides, would have to be made.
IIRC again there are upper limits even for completely new pads. These are imposed by the acoustic loads on nearby structures and adjacent populated areas. Whilst it may take you quite a few more boosters to do it and you might, with lots of money expended, build Pad C, strengthen various on base structures etc. the upper limit is the fact that you don't want broken windows and claims for hearing damage in Cocoa Beach!
One obstacle to adding more boosters is adding the necessary cross-beam structure in the core stage's intertank. In the case of adding two Delta IV cores, how would the forward domes attach to the Ares core? Unless the tankage is stretched, there's no way they could reach the intertank area.
I saw some imagery a little while back, and it looks like they would just attach Delta-IV Cores to the tanking area directly.The Delta-IV Cores don't have the same Thrust Oscillation issues as the SRBs do, so a brace between the mountings isn't required. Also, the pressurized Tanking is able to handle quite a bit of side-load anyway (with a few suitable modifications) - think about the Aft Orbiter mounts on Shuttle today.Ross.
Off topic, but where is the dynamic load from the SRB's provided to the core of the Ariane-V? Is it the Fwd or Aft attachment?Ross.