Next Steps into Applied Energistics: P2Ps and Subnetworks 

As long as you use Quartz fiber cable to keep the channels from being shared, you’ll have no problem powering the subnet with the mainnet

Without knowing the context, my best guess as to the problem you’re running into is the fact that interfaces and the like can only carry 8 channels through them, as though they were a normal cable. As you can see I have each group of interfaces separated into rows of 8 for that reason. Without more context it’s very possible you’ve got a different issue though.