Trying to run two simultaneous loops that do the same task with different delay cycles
The Arduino Simple Timer library (and other similar timer libraries) are made for exactly this kind of application. Your setup() routine calls the SimpleTimer library, naming a callback-function and how often it should get called. Your loop function must then call timer.run() reasonably frequently (where 
reasonably frequently" depends on the timing accuracy and jitter tolerance of your application). timer.run() will call each of your callback-functions at its appropriate time. You write the callback-functions to do whatever you need them to do when the timer calls them.

There are one-shot timers and repeating timers - it sounds like your application could one of each per LED: One that fires periodically, to to call a turn-on-LED12 function every (ON_TIME_12+OFF_TIME_12) ms, and a one-shot that calls a turn-off-LED12 function, started by the turn-on-LED_12 function for (ON_TIME_12) msec. Same for LED13, ( ..., etc.)

Your loop function is free to do whatever else, as long as it calls timer.run() frequently enough that your application can meet its timing requirements.