Hi Michelle,
Welcome to Purrs. One tray for 2 cats can get pretty congested with cat waste and the fact that waste is kicked on to the surrounding floor or stood in is not surprising.
There is one very simple thing that you can do immediately. The feline experts largely agree that the rule of thumb for cat litter trays is one per cat plus one extra if it can be accomodated. We have 7 cats and 8 trays because in the early days we had a similar problem.
The siting of the trays, type of trays and cat litter are also important.
1. Siting
Think about ourselves. We like a bit of privacy when visiting the loo. Cats are no different so a relatively quiet and secluded spot is the best bet. Somewhere where they are not on public display. Do not put the trays together. Try to create a little space. It is inadvisable to place the trays near their food or drinking water.
Types of tray
Essentially there are 3 types - the open tray, the trays with a rim to prevent litter being spread and the covered tray (with or without flap).
The tray should be of sufficient size. The open trays and trays with a rim come in the traditional small, medium and large sizes. The covered tray is usually one size.
Be aware that not all cats like covered trays, especially if it has a flap. However, the entrance flap can usually be removed. The reason is due to a fear of entrapment in a confined space, especilaly if they are nervous cats. Other cats take to them readily. It is a case of trial and error. Your cats are very young so they may take to this being of tender years.
I have just invested in open trays with rims. You can buy these at Pets At Home - not a chain that I like but I haven't seen them in the independent pet stores. I was recommended these trays by a boarding / rescue cattery owner who has them in her cattery and they are good. It may be your best bet.
Cat Litter
Whilst important, I don't think this has anything to do with your question but it is trial and error based on watching your cats reaction to the litter. I will not elaborate on that and, besides, I do not have as good a knowledge as others on Purrs on the subject of litter.
The secret is putting yourself in the cats' position and trying to look at things through their eyes rather than yours. Sometimes I simply lie stomach down on the floor to try to see things from their perspective. I hope this helps.