1999 Volkswagen Beetle cooling fans

There are two cooling radiator fans. One small one and one larger one. Do they run at the same time during normal cooling temperatures or do they back each other up. When my thermostat meets temperature(engine temp warms up), the small one comes on. I cannot get the big one to come on. Something wrong? Or haven't I let it heat up long enough? I haven't let it run long for fear of overheating the engine if the 2nd is suppose to come on. Perplexing. Thank you.