I need to buy a replacement transmitter for the two collar system. How do I know which one works with my collars? I can't remember which I had purchased.

Can you please provide us the item number you have then we can assist you better.

I need the transmitter that works for the 1902 two collar. Which one is compatible?

You can check the given link for your item.

https://www.dogtrainerstore.com/products/1902ncp?variant=58843518155 

I would like to replace the transmitter for my dogtra arc 122484 2 dog system .can you please help me out

You need to provide the correct item# of the product you have.