Sims 2 installation

You must install the expansion packs in a semi-fluid order. That is, there was an order they were released and for best results, you should install them in the same order.

http://compsimgames.about.com/od/expansionpacks/qt/sims2_exporder.htm

The above site is the order of the expansions. Keep in mind, it starts at the top and goes down (oldest at top, newest at bottom). You must use the latest most expansion pack's disc to play. That means, if the last expansion pack you have is "Seasons", you will need to use that disc to play the game.

Due to copyright laws, I'll be unable to assist you in how to play the game without the need of the discs but in essence, the above information will at least get the game installed with the expansions in the proper order. You and your friend may just be unable to play the game at the same time.

I don't think it matters which order you instal the packs in, but it does matter which order they were released in. Whichever pack you have installed which was released latest is the one you will need to own as that is the disc you will need to use.
I believe your friend's packs contain the latest released ones in your combined collection so you will not be able to swap back and still play.
However, you can find some codings or cracks which will enable you to play the game without a disc.