When I used EU for a mining laser, it took 512 eu/t per precharger and it just kept the buffer full. I'm not sure what the conversion ratio is, but you may be maxing out your connection with the energy conduit. They are 10k RF/t per connection.
It is only 10k per connection. The conduit itself is not limited to that on throughput. I thought KL said they were basically unlimited throughput.
After some testing, this is what I found. The MFR laser takes 5k RF/t per precharger to run at max power. That is a total of 20k RF/t. A single conduit can feed it with one connection per precharger since they are only 5k each. The conduit can carry the 20k with no problem. You have to attach the conduit to 2 faces of an energy cell to get enough power to run it full speed. You cannot use anything but a Resonant energy cell. Even if you used 5 faces as output from a redstone energy cell, you would only be running the laser at half power. With just 2 connections, the prechargers hold steady but won't fill their buffers since it is using the full 20k RF/t. If you make a 3rd connection the the energy cell, the precharger buffers fill and the tops of the pre charger turn red. The laser itself will never fill its buffer. It will also never draw more than 20k RF/t no matter how many connections you give it.
After rereading the original post, the answer is simple. You aren't making enough power to run 2 lasers. You will need all 8 2500 RF/t reactors just to run a single laser at max speed. I would guess they are less efficient when not fully powered, so splitting your already insufficient power in half would have less output.