*cough* Recount *cough*
Edit: There seems to be some confusion on what/how to clear these instances so your lockout sharing alt doesn't get saved.
For ICC, clear everything on 10 man NORMAL mode up to LK. So now you have a 11/12 lockout (or 10/12 if you skipped Valithria). Make sure the alt stays outside the instance, get the group together, and once you are on the character you want to kill LK on, fly to ICC and make sure it is set to 25 man NORMAL when you enter. You'll get the "This instance is in progress, 10/12 blah blah," press OK. Switch to 25 HEROIC and you get another notification. Press OK, kill LK, weep because Invincible didn't drop. The alt that shares lockouts will not be saved if you do this.
For Ulduar, clear everything on 10 man. 100x faster if you do this on a rogue since you can use the Shadowstep exploit posted here to skip everything after Kologarn and just kill Vezax immediately. So now you have a 4/13 or 11/13 lockout (or w/e, forget the number of bosses there are in there). Get the group together, and make sure the raid size is set to 25 man when you enter. You'll get the instance in progress notification, press OK, kill Yogg, weep because Mimiron's Head didn't drop again.