What is the difference in sound between the RS410 verses 2 RS210 cabinets?
The main difference in the cabinets is that the 210 uses a coaxial tweeter and the 410 uses a separate tweeter. With two RS210 you will get more versatility because you will be able to place them side by side or you can stack them for less space on the floor in small clubs. This will give you more bite and presence because the cab on the floor will give you more bass and the one above will give the treble. Two RS210 will have more the presence and more high frequencies because of the two tweeters.
The RS410 should have a bit more bass because of the bigger size cabinet but only one tweeter.
You have to think about the impedance as well. With two RS210 you will have a 4 ohm load, the RS410 is an 8 ohm load. You can use the RS410 and add one RS210 to give you a 4 ohm load which will give you more power and the largest frequency range.
Here is the information about the cab specs: