Motherboards can't be damaged by having a "too strong" power supply, the PSU only delivers the energy that your components draw.
A 900W rating means that's the maximum that this particular PSU can deliver, but if your components have a combined draw of 300W, that's all they get.
For using one power cable for two disks, you may use what's called Internal Power Supply Cable Splitter
or Power Supply Y Cable
from your local electronics shop. However, ensure that your PSU is up to the task.