Prague powder #1 is 6.25% Sodium Nitrite bonded to salt as a carrier. (One ounce of Nitrite to one pound of salt). There are gobs of different 'cures' that contain Sodium Nitrite and will often contain other flavorings as well. That's all well and good if you realize what you are using and what it is made up of. Personally I like to just purchase plain ole Prague Powder #1 and fine tune my spices as I see fit.
Prague Powder #2 however contains the 6.25% Sodium Nitrite, Salt, but also adds in 4% Sodium NITRATE. (One ounce of Nitrite, one pound of salt, .64 ounces of Nitrate). The Sodium Nitrate is a time release preservative that, over time, breaks down and converts into Sodium Nitrite. #2 is needed for long term curing type products that will never be cooked.
These two can NOT be interchanged. As a matter of fact, the #2 does have a shelf life, so if you are making a product using #2, don't buy a bunch of it as it will slowly be converting the Nitrate to Nitrite on the shelf and if it sits for very long it may NOT contain the necessary amount of Nitrate when you finally do use more of it.