I've bonded sows a few times.... my general method is to first move the newbie into their own cage to do a quarantine period and make sure there are no infectious diseases that can be passed between them. After this, when I have several hours to spend making sure things go smooth, I will introduce them in a neutral space (I actually have put towels down in my bathtub the last two times- they don't know it, there are no corners to be backed into, and I can keep a good eye on what is going on!) I put some food in to give them something to interest them and put each pig at an opposite end... they take some time to meet up in the middle where the food is. I watch them to make sure that there are not going to be any fights. Typically there are some dominance behaviors (rumbling, chasing, mounting, maybe some carefully-judged nips to put the more submissive pig in their place.) Pigs that have accepted each other as friends will often do some mutual grooming of each other's ears and faces and may sit close together once they have decided they want to be friends. There are some more detailed guides about introducing/bonding pigs at the top of the page, as well as some videos of normal dominance behaviors. Once the pigs have accepted each other, I will wash down the entire cage I want them to be in (so that it doesn't smell like the original owner's territory) and then will put them there together and keep an eye to make sure things stay peaceful. I've luckily never had a problem matching up pigs, though I did try to personality-match as much as I could (for example, for my very bossy, domineering pig I made sure that her new friend was very submissive and not inclined to be the boss!) As for buying multiple items, it really depends... pigs will tend to chase each other out of hideys no matter how many you have (the best hidey is the one that someone else is sleeping in, of course!), but it's good to have a resting spot for both. Water bottles and food bottles... I've never had to have more than one water bottle. Sometimes the dominant pig will butt in and steal the water bottle from the subordinate pig, but the subordinate pig just waits in line again and no one goes thirsty. Right now I do have two food bowls, but again, it depends on the pigs... I have gotten used to having two bowls because my previous pig was a food hoarder (she would drag the bowl into her pigloo and sleep basically on it so no one else could have it!), but now that she has passed on and neither of my current pair is prone to hogging all the food I could probably get by with just one. My only other advice is to triple check the gender of the new pig to be positive that it is another female- pet stores have been known to make mistakes at times and you want to be sure that you are not getting a male by mistake!
Hope this helps a bit and good luck with your introductions!
