What precisely do you mean by a multi-clip? Are you talking about a mutli-camera shoot? As for the 720 and 1080 mix, if you put them on the same timeline, one or the other will require rendering. However, if the one entire section needs to be one or the other, you could always edit it seperately and just bring it in at the end for the locked picture (but I'm not imaging that to be the case).
I'm looking into the best workflow for using FCP multiclip to mix HDV footage shot on the XHA1 and footage on a HVX. Here's what I've found....
disclaimer: I am doing wedding videos, and my customers typically don't know anything about compression, hdv color space limitations, etc.
Also, I know that you can edit both types on the same FCP 2 timeline, but I am talking specfically about using FCP's multiclip feature.
Workflow #1 - The poor man's (or iMac workflow). Time > Money
Shoot 1080i, 24 fps HDV with the Canon. Shoot 720p, 24fps native with the HVX (FS-100). Downconvert the DVCProHD content to HDV using FCP's Compressor application. I let mine cook overnight, and it does the job. I limit this conversion to only the footage that I know I will be using (primarily ceremony). Once converted, use Multiclip with the HDV versions of both clips. In my experience, converting the DVCProHD to HDV was a lot faster than the other way around, not to mention it consumes less drive space.
Workflow #2 - Aja Kona. Money > Time
Use a Mac Pro, get a Aja Kona LHe card. Capture the HDV from the Canon to DVCProHD in real time using the Kona card. Make sure that you ingest the Canon footage to match your HVX footage.
All in all...I am happy with both cameras. It will be nice is Apple can get Multiclip to work with different formats like they have done on the timeline. That's probably a tall order though (IOTW, don't hold your breathe).
For what its worth....for my HD customers, I always end up downconverting the final edit to a widescreen standard definition DVD. But, they really like it when I also output their weddings to HD WMV file on the disk.
Budget constraints prevent me from having 2 HVX's. I wish I could, but its cost prohibitive. Perhaps, it would be nice to have a second XH-A1, but I really like having the ability to overcrank. I suspect that there are quite a few budget minded people on this site that may have an HVX and are looking at the XH-A1 as a second camera.
I understand the differences in HDV vs DVCProHD, but I'm not sold on upconverting to DVCProHD in software for my audience. On my mac it takes way longer to upconvert. And, despite the lack of I frames, I've never had a problem editing HDV in FCP Studio 2. Further, I've yet to have a customer ask if there footage was 4:2:0 vs. 4:2:2. Most of them are just happy to have their slice of life preserved onto plastic.