No problem. There are a few things you can do here, depending on what you have:
If you have SCP installed, go to your FS2 folder and add a new folder with a campaign's name as the title. Once you've done this, either copy/paste the VP from the ZIP to the folder, or, if the campaign came without a VP (i.e. in loose files), create a new folder inside the campaign folder named
data. Inside
data, add the respective files to their own folders (missions in
missions, models in
models, etc.). Repeat for each campaign, and then use the MOD option in the FSO launcher to choose a campaign.
If you're not using SCP, this may be a bit more of a problem. If none of the campaigns have overwriting tables, then you should be fine: simply copy/paste the missions into the
Freespace2/data/missions folder or in the VP in the
Freespace2 folder and you should be fine. If any of the campaigns have mods (i.e. new ships or edited tables), then you'll have to play one at a time.
What campaigns do you have downloaded?
