That's the biggest downfall for double headers. However, you could get around this possibly with some ingenious camera techniques. Using last year's semi-finalists as an example, have Leeds/Saints fans at one side of the ground and Hull/Wakey fans the other and change which side the camera faces for the second game.

Example:

LLLLLLLLLLLLLLLSSSSSSSSSSSSS
L-------------------------------------S
L-------------------------------------S
L-------------------------------------S
H------------------------------------W
H------------------------------------W
H------------------------------------W
HHHHHHHHHHHWWWWWWWWWW

Shouldn't be impossible to set-up, no?

COM stadium hols 48000 divide that in2 four and you get 12000 im sure if we could manage to fill it if we had the well supported teams there.

Should point out that in Union, in 2007 and 2008, the Semi Finals of the EDF Energy Cup (The main knockout cup for English and Welsh teams) were played as a double header at the Millennium Stadium

According to Wiki, crowds were:
2007 - 40293
2008 - 41018

The teams that competed in those double headers were:
2007 - Leicester, Sale, Cardiff, Ospreys
2008 - Leicester, Wasps, Ospreys, Saracens

It got 50,000 the year before that, also.

I don't get how you can say a 40-50,000 crowd didn't work. That's not far off the actual final attendance for that competition.

40-50,000 would fill the CoM. And because CoM and MS are tiered stadiums, you can keep the top tier closed to make it look fuller if that is the desired effect.

When was the last time the combined semi-final attendance was above 40,000?

Personally i think the double header is a great concept.
I just can't imagine many neutrals attending if they have to travel to Cardiff.

I think an Elland Rd or Reebok Stadium double header would be fantastic.

If celtic got to the semi finals, i dont think they would move the game to the Millenium, as there would be no point, to put what? 10k fans in dependent on the away team. IMO i think they would put that game to one of the neighbouring Rugby Union grounds, or they will play at Swansea or Cardiff, which i dont think Swansea would be a bad ground to play it at.

probably too many events already as said by another poster.

concentrate on the new MM and let that develop.

if the WTB want an event they can bid for that again

I reckon this is a great idea. Ive just had my stag do in Cardiff as i loved it down there for the challenge cup finals. Had a great weekend and the pubs, people and atmosphere of the place is sheer rugby. I happen to dislike rugby union a lot but I must admit it was great to be somewhere and not be blasted with premiership football from every angle. Itwas also a nice surprise to see Celtic kits feature very prominently in the sports shops as well as a few aussie club shirts.

If they do play the semis at the millenium, i'll be there. (as long as Leigh make it!)

I have to say that you are totally wrong to say that the MS and the neutrals could not fill the stadium for MM. The 12 SL teams could not fill the stadium. Blame the fans for not travelling if you want someone to put the fault on. I would like to know if you actually went to the game or are one of these people who blames everyone but himself.

Have a lok at the television guide for the 4th round and you will see that the BBC are showing a double header

The key for the double header at the semi's is that you get to see the team your club will face if they get to the final so it would have a lot more draw to it then MM where there are just 2 league points on offer. I like the Idea of the COM perhaps Bramall Lane could be could be big enough, I feel Cardiff maybe just out of reach unless some very creative marketing was on offer for passholders (2 tickets for £20) or something. Again these deals then help clubs marketing season tickets each year.