They can see one another.. they may flare for a few days, but they tend to calm down. Plus, a flare session a day keeps them healthy. You can place large plants on a side so it breaks up their view, but don't stress if they can still see each other in places - they actually prefer to see another fish, as these domesticated ones are quite social. They just like to pump their chests now and again.. silly males :P
As far as dividers.. Petsmart sells dividers for 10 gallons, but they are a pain!
Home made ones are from the plastic crafting mesh (crafting section of store - Walmart sells cheap), plastic report binders (can be hard to find - I found mine at Dollar General).. sometimes aquarium sealant is needed to keep them extra sturdy.
Here are two links that show two different ways to make them.. others may have better ideas. You can also see if you know someone who can cut plexi glass to fit the tank, and drill small holes in it for the water to go through..
Just remember to make the top as high as you can - can cut a section of some left over divider sections to make a T at the top to avoid jumping - may also have to keep the water level a bit lower.