Yes, this appears to be the case. I installed the game once, from one of the ISO images. Then I installed the XP patch. Now I "insert" the appropriate CD by mounting its image using MagicISO, and I can then play that campaign.
It works the same way for Tiberian Dawn. I haven't tried Tiberian Sun yet, but that's next.
[Edit] Update: Tiberian Sun is by far the easiest of the three to install - just unzip the folder into Program Files, and use the shortcut to launch the game. No ISO mounting or patches necessary (at least for XP Pro). James Earl Jones in the opening scene gives some indication of the higher budget for this one! The graphics are better too - quite similar to Starcraft, which makes sense as they are roughly the same age.

If memory serves, the only content different on the 2 discs was the videos. It should be possible to pull them from the disc and point the game at them manually

Aaaaah Tiberian Sun. I remember the high hopes, the anticipation, the tempting screenshots ... and then people played it and got annoyed by the changes. It's still a good game but it lacked the style and charm of C&C and RA1.

Tib sun allows for higher resolution, 800 x 600... i'd much rather tib sun over starcraft, not sure why to be honest, i just always found it more apealing and keep coming back for more.

That's what they want you to do. ;)
Glad it helped.

And if you manually edit ini file, TSun supports pretty much any resolution. Though it does start to demand a lot from CPU fast on high resolutions as those voxel graphics do not enjoy any hw acceleration. But on modern computers 1600x1200 or even widescreen resolutions should be playable.

Aw, now it's free I feel bad about buying it over a decade ago! :P
On the plus side, the online community is sure to get a jump.

anyone made them run on win7 64bit?

Great set of games. Perfect for the laptop during slow days at work.

Awesome! I tried it last night for a few hours and it works like a charm on my pc at 1680 x 1050, No slowdown at all :D
Im enjoying the game so much again im thinking of going back to modding it to fix it up for the new resolution, ie. longer firing range for most units, health, cost adjustments & ectra.... basically to allow for larger battles & more incentive to purchase masses of infantry over vehicles (similar to the warhammer games).

I've been meaning to mod Tiberian Sun units as well, but I don't think it has such a good editor as the first C&C game has.

Actually the Rules.ini editing in TibSun is a lot easier than it was editing the main exe for the first C&C game.

As what lordfirefox said, the tools available made it dead easy to mod... its the only reason why i use to edit it.
The first C&C editor was too buggy, as it directly edited the .exe file if you didn't have the 95 edition and you couldn't change everything from what i remember.

I remember this editor with a putrid yellow/brown background that could change everything and it worked without flaws.