Not all dryers have a second lint trap.
The majority of dryers have one lint trap which can be near the dryer drum in front or at top where you pull it out to clean.
You should clean your dryer lint trap after every use of the dryer as buildup of dryer lint in teh trap can cause the dryer to work harder and could even start a fire.
You should also clean the dryer vent of any lint as well.
The dryers that often have a second lint trap are dryers that have a vent that is longer than 20 to 30 feet.
There is a second lint trap for ensuite condo dryers.
Cleaning both dryer lint traps assists with efficiency and reduces the risk for a fire.
The second dryer lint trap would be mounted inline of the dryer vent tubing which could be right above your dryer or even in the basement or in the crawlspace or attic depending on where the dryer vents out too.