2 sharing the same volume in an enclosure (be it sealed or ported) is doable and will not inherently damage the speakers, and won't automatically sound bad either. JL has sold boxes like this so it's not a "do it and you're a loser" type of deal.
However, the problem with it is that you need more volume than you would for one sub. This could
potentially lead to over excursion and damage to the subs because of the loss in suspension control, particularly in vented enclosures. This possibility depends on how much power you are sending the subs, and the size of the airspace. Compound loaded enclosures do not apply to this.
So, it's not all that hard to put a board down the middle of the box and create seperate sealed chambers, and it is good insurance regardless of the type of box. You would need to add a second vent in a ported enclosure.
Two subs will only be louder than one sub if you give them both a greater total amount of power as you did the first sub in the beginning. This is because adding another sub to one that is recieving 100 watts will divide that power between them, now making 2 subs with 50w each. To increase the sound output over the one that was taking 100w, you will need to add more power. This is assuming you're using a single channel amp.
I can't imagine anybody wanting more than one well powered sub in a sealed box, especially in trucks with a teeny cab like ours. Properly done single sub output can exceed the threshold of pain.
To those of you unsure on how or why to properly construct a box, see number 110 in the directory on the side of this page.
http://www.bcae1.com/
It is a good site to bookmark if you are building your own system and have questions about the process.
I didn't mean for the post to be so long winded, but there is a tremendous amount of depth to the subject. You never know it all, no matter how long you've been doing it.