Multiple dog households are wonderful IMO as long as you can handle it, which I'm sure you would be able to.

Trina used to be an only dog for a while and she started to get older. She was overweight, has hip displaysia. Well, when we got a Angie, who was about 5, 6 months old, it really livened her up. She lost alot of weight playing with her too. Now Trina will be 12 this July and she is still running around the pool like a young dog catching small food pieces you throw at her with astonishing accuracy and has yet to slow down and act her age.

When we got Rosie as a puppy, Angie really helped to teach her and socialize her. Like on the fourth of July, Rosie was afraid of the fireworks and the noise. But we went outside and went to a neighbors house and because Angie and Lacy (the neighbors labrador) weren't freaking out, Rosie calmed down. She isn't afraid of any loud noises now, thunder fireworks, gunshots....She even gets in the way of the vacuum and you can vacuum her with the hose attatchment.

First off, will Jenny get bad ideas if the other dog isn't as well trained as her? Say, it doesn't come when called, will Jenny stop coming when called?
Nope, that deffinitly won't be an issue. As long as Jenny has a reliable recall even with distractions, it won't happen. As mentioned above, it will actually work the other way. If you call Jenny and she comes, the new dog is likely to follow, thus helping you teach the new dog to come when called too!

What changes? Like what is the difference, walking one dog compared to walking two?
Once you get them situated, it's much like just walking one dog as long as they both heel good. Just make sure you teach the new dog to heel before you walk them together. They may get a bit excited the first time going out together, but they'll get used to it. The only difference is that people tend to look at you funny when you walk more than one dog. You should see the looks I get when I am walking two huge german shepherds and some little 18 pound mutt

Will it not be as attatched to me as Jenny is?
Once you develop the bond, it will be just at attatched to you as Jenny. It's important that each dog get it's own special time alone with you. This can be accomplished through a one on one walk or a nice, calm grooming session.

Now, whenever we watch a movie, Jenny will quietly lie on her bed. If we got another dog, would they both lie down, or would they be up playing with each other?
Most likely, the new dog will lay down too. You and Jenny are going to be setting the mood. Once the new dog gets situated into the house, it will learn when it's time to be hyper and crazy, and when it's time to settle down. When I first got Angie and it was bed time, she thought it was just too cool that I was laying on the bed and she thought it was time for her to jump all over me and lick me to death. But by the third night, she learned that bedtime was time to go to sleep. Same with Rosie. When I first got her and it was the first night in bed, she thought it was playtime. But she soon learned through me and Angie that bedtime is for sleeping. And just to clarify, both of those dogs sleep in the bed with me lol.

How do you keep the fastest eater from fighting for the others remaining food? Or would they not be able to both eat in the kitchen? (my mom isn't fond of the idea of them not eating in the kitchen, because it would ruin the carpet)
Just do different ends of the kitchen and make sure you police them while they eat. This way, if one gets done faster, you are there to make sure he/she doesn't try to steal somebody elses food. Mine eat in different rooms, but no closed doors. Rosie eats under the table in the family room, Trina eats in the kitchen in front of the refridgerator and Angie eats on the couch in the front room (yes, on the couch lol). The one who finishes first will eventually learn not to bother the other and you won't have to watch them like a hawk.

Would they fight over the 'best' seat in front of the fire?
Probably not. They will either share it, or one will have it and the other will find another spot to lay down. Because it was jenny's spot to begin with, it is most likely that it will remain Jenny's spot and the other dog will find somewhere else. Rosie and Angie both share sleeping spots sometimes, you will find both of them laying on the couch or recliner, I have some really cute pics of them on the couch together.

How do you tell the difference between play, fighting for dominence, and 'real' fighting?
The majority of the time, with real fighting or fighting for dominance, the hair on the back will stand up. With play fighting, there will be alot of play bowing, high pitched barks, and jumping on each other. Although dogs can be extremely noisy during play with snarls and growls, the snarl you hear during a real fight is different....It's hard to explain, but you will know it when you hear it.

And a biggie, because I've heard a lot of different opinions on this one: Are two dogs not much more work (than one dog is) or a LOT more work (than one dog is) ?
The only time two dogs is double trouble is when the dogs have behavioral issues and aren't trained. You already have one dog who knows the house rules and is well trained, so you probably won't have any issues with another dog asimilating into the family. Jealousy issues can sometimes cause problems, but that is fairly easily reversed. Rosie didn't like it when I payed attention to Angie so what I did was I made it a good thing for rosie when angie got attention instead of a bad thing (like being ignored). So when I was petting angie, Rosie also got praised, petted or was given a treat.