Ah not so much. They are unfortunately two different variations of the DVCPro HD codec. They won't cut together w/o requiring a render. I'm not sure what you have access to, but you could always lay one of them back to tape w/ an AJ-HD1200A (or similar) to conform one format to the other. Not ideal, but if you've got a lot of material and a long edit infront of you, might be worth the headache now rather than later.
Oh, also, you didn't say what you shot with. If you shot with tape you could simply use an offline codec, bring both formats in at the same offline codec and cut it then. But if you used the HVX 200, well then not so much.
I haven't tried exactly what you are describing with 1080i and 720p, but you might want to take a look at the Advanced Format Conversion settings in Compressor, which include both DVCPro HD 1080i and 720p. I have used the format conversion feature to downconvert 720p footage, and the Compressor manual indicates that deinterlacing is supported. It might be worth a try. Let us know if it works.