I'm working on a multiclip project that was shot with one AVCHD camera and one SD camera. I found out you can't mix two formats in a multiclip sequence so that means I have to either Upscaling SD or Downscaling HD. I figured the second option makes more sense since the footage will likely be going out to DVD. (yes I realize either way something is going to look not as good, i've accepted that.)
As such I was curious as to what the workflow would be. I figure I should use Compressor but wasn't sure what settings I should use (specifically the frame size to ensure the footage comes out 16:9).
Also, I've brought the AVCHD footage in as Pro Res (Proxy), but wasn't sure if I should bring it in at the full pro-res then convert to SD or if the Proxy would be good enough?
You didn't mention if you SD was anamorphic. If it isn't, then you are going to have some aspect ratio issues. If your SD is 4:3 then convert your AVCHD to whatever the codec is for the SD and do edge cropping. In other words, if your SD is miniDV then convert your AVCHD to whatever DV codec your NLE uses and edge crop. If you have used full frame then you have bigger issues trying to pan and scan this stuff around or coming up with some other method to see what you need to see in the frame.
Once you have the two clips as the same aspect and codec, multiclip should not be an issue.