Sorry for the delay. I try to focus the engineers here on problems/issues/questions with our tools and SDKs. So general "programming advice" I try to leave to the community. But, this has been sitting for a while, so I checked with someone internally, asked them to take a look. Here's what I got back.
My recommendation to this user is to not do this at all. The overhead of writing all the data out to memory in transform feedback, then reading it all back in again on the subsequent draw, plus the possibility of a pipeline stall as a result will dwarf the cost of just running the vertex shader twice unless the shader is extremely complex.
Hope this helps.