No, you can't just switch discs to play multiplayer. Believe me I tried. The only option that shows up to is to install disc 2 and when I back out of that the only thing that shows up is Failed to install and please insert play disc. The question is, why does Microsoft and 343 feel the need to screw us over by requiring you to install an 8 GB download when not all Xbox 360 systems can handle that out of the box? Especially given that other games in the FPS genre (I'm looking at Call of Duty which releases another title next week and is therefore competition and the previous Halo games) can do it on one disc! Better yet, why do they feel the need for 2 discs in the first place? Or how can you play Mass Effect 3's multiplayer without having to make a 8 GB download regardless of which disc is in?
There is no reason why the multiplayer should have to be downloaded. There is no reason why the multiplayer should be on a separate disc to begin with. I'm taking my Limited Edition of Halo 4 back to the store and I will never EVER play this game again because I am not going to support a game that is so overly demanding on my system for no reason whatsoever. I'll go back to playing Halo Reach which I can play without having to download a 8 GB multiplayer add-on.
And for those of you that think that this is not a big deal, this is a HUGE deal that everyone should be vocal about. If every game started having parts of it that couldn't be played without a multi-GB download/add-on, imagine how much space your hard drive is going to lose. This is not a post-release add-on either, but an integral part of the game. A part of the game that should be on the PLAY disc. Other FPS and TPS games can make the core game onto one disc, why can't 343?
And just so that it is known, this is not in anyway a anti-Halo post. I preordered the limited edition of Halo 4 and have spent many hours on Halo, 2, 3, ODST and Reach. I would have loved to play this game, but what's the point of owning a game I can't fully enjoy.