Hello,
I'm trying to install a second graphics card in my PC. The problem is, there are only two 6 pin(I think that's the right word) power connectors for my PSU. I have two IDE power -> 6 pin adapters. If I put both of those adapters on the same IDE power line that is coming from my PSU, will it harm my PC in any way?

You need to determine what the output of your power supply is, and the power requirements of the cards, motherboard, CPU, disc drives, memory, etc. A lot of systems do not have adequate power for what you want to do, so you may need to get a better power supply.