Are Fire Prisms better together, or apart? I'm specifically talking about two Fire Prisms here, and using their ability to combine the beams.
Here is a shot from my most recent game against Space Marines. My two Fire Prisms are on opposite sides of the board to increase the area they can affect with their deadly Prism Cannon. In this way, each can contribute to the beam of the other while maximizing the area of control they exert on the battlefield.
Perhaps this is what makes the Fire Prism so deadly, confusing, and frustrating to face; with the ability to move as a fast skimmer, and dropping a template with such destructive capability. It is no wonder the Fire Prism is my vehicle of choice. Combining the beam of the Prism Cannons makes the weapon even more deadly for a couple of reasons. First, is it allows you to shoot a large blast template that is S:6 AP: 3 (or S7 AP 2) or a small blast template that is S: 10 and AP: 1. To further frustrate your opponents, dual fire prisms that combine beams means the attack has different ways of getting to the target.
When it comes to deploying the Fire Prisms, my preference is to keep them far apart to effectively "put pressure" on as much as the battlefield as you can. When using a Fire Prism (in a purely attack role) it is akin to wielding two blades at once. If used correctly you can use the area and angles of the beams to get clear shots on your targets and avoid the dreaded cover save. Remember, there is no limit to the range to combine the beams, meaning as long as you have line of sight from one Prism to another on the battlefield, then you can do it from anywhere.
What also is annoying and frustrating to your opponent when using two Fire Prisms is while they are separated means that if you opponent wants to deal with both of the grav tanks, they will need to divide their units in most cases. Slower forces will have a tough time deciding to go after the distant Prism, and if they don't, it will just keep firing and contributing the beam all game.
Even if one Prism goes down, or loses the cannon, the second one can still fire.