Overclocking the core will make the GPU hotter.
You have a 65 degree limit, if it gets there, the power will be cut to stay at that temp.
Since both cards can have different temperatures with the same settings, you should set the 1 that is holding back a bit higher, lets say 70 degrees.
So yes, temp limit will ultimate ( if it gets around the set temp ) lower the power.
And my advice, don't use auto fan, set them yourself
If it's get warmer, a little bit more, but with your cards, a fan speed of 65 70 % will keep them cool enough, just find the setting that suits you, more fan is more noise