Though I agree with Abbi.. (Little miss word of caution hehe <3 ) .. I also tend to think how boring things can get doing the same thing over and over, adding that extra zone will not only teach us things for the other, but keep us fresher. Also, how often have you been on a run or whatever and found yourself saying, "Hey, this reminds me of and we did this or that and it worked great ..... " so learning too by doing more than the one would be an added benefit.
(Though as Abbi said about getting the hang of things, I'm more so thinking getting that other group together and then implementing the idea of swapping weeks)
I don't think there is any worry about working as a unit, people won't be swapping in and out unless someone isn't there, and for the second group it would be the same way. And running the instances afterwards only reinforces that same "oneness".
Also, I think because of lockouts perhaps it's better for alts to fill in from the other group when possible (more so because its the same person behind the char, so learning the play styles of the other group will only benefit for bigger raids way on down the line) .. IF there isn't another not in a group to fill in. <--- though that could be a loot issue when you have someone who is there all the time vs someone filling in. Regular members of the group should get preference over someone who is a fill in, maybe that could also go along the lines of alt loot.
I only say this because there should be some sort of benefit or .. I can't think of the right word, but for those who have been there through all or most of it over someone who hasn't put in the time. From those I know in the first group, none of us are the greedy sort and always help each other in any way we can, and I don't want it to sound like we would be that way, it's more of a acknowledgement to the one who has been there, not anything against the one who would be filling in. (I hope that is making sense)
As far as alts go, we talked about this before and personally I think any main gets dibs over an alt. The only exception would be in the case of that main not needing it where an alt might, so the alt might get a need roll over a greed for the others in the group that couldn't use it. I think so long as this is stipulated ahead of time it wouldn't even become an issue.
Just some things I'm thinking.. sure a lot of this is a bit further on down the line, but thinking ahead can help keep issues at bay.
Also thinking as well.. what about weekends(days). I know some can't be here at all on weekends, but for some weeknights are inconvenient and also we have some European types who've expressed interest but even as early as it is for us, 4:30 server, it's a late start for them, so perhaps like noon server might work well.
I have been encouraging people to post their interest here instead of just sending tells so that you know better how many would like to participate and are raid ready. (or as some have, want to participate once they are)
I was going to say just my 2 cents, but it was more like my 20 .. :P lol