Using two Arduino dimmer's on two lamps using one eletrical grid
This can get unexpectedly complicated. 

The way your dimmer module works by waiting for the AC power signal to cross zero volts and then turning on the power at some delay after that. 

Depending on where you live and how your house is wired, the two lights you are controlling may or may not be connected to the same phase. This means they may or may not cross zero at the same time.

If you are sure that the two dimmers are on the same phase (if, for example, they are both plugged into the same outlet), then my guess as to what is happening goes something like this...


Both dimmers see the same zero crossing at the same moment. 
The two zero crossing connected pins change at the same moment. 
The pin with the higher interrupt priority triggers and interrupt. 
The 1st interrupt handler executes. While it is executing, the 2nd handler waits.
The 1st interrupt handler does turn on the light at the right time and then turns it off, which takes some time. 
The 1st interrupt handler finishes and returns.  
The 2nd handler now gets to run, but by now things have changed. We are not at the zero crossing anymore, we are after the place in the wave where the 1st handler happened to be finished. 
The 2nd handler runs and turns on the light at whatever place in the wave it happens to be now. This takes some time. 
The 2nd handler returns. 


You can see that wacky and timing dependent stuff can now happen.

Probably the most robust way to fix such that it will work with any outlets with any phase is to make the interrupt handlers run very quickly to minimize the amount they can interfere with each other. One way to do this would be to replace the cycle-burning delayMicroseconds() delays with timers and interrupts. 

If timers and interrupts are more effort than you are willing to put into this project (they are super useful, but can take a lot of effort to get working), then you could hack something easier together if you knew that both dimmers would be on circuits with the same phase. You could have a single interrupt handler for both dimmers. This handler would figure out what sequence to turn both dimmers on and off and the delays between the steps and then control both dimmers in a single flow of execution. Make sense?