How can I run two loops simultaneously on an Arduino Uno?
You can make the sensor return data faster by lowering it's accuracy.

tsl.setIntegrationTime(TSL2561_INTEGRATIONTIME_13MS);      /* fast but low resolution */


You can also modify the library to not power-down the sensor after each reading. Just open TSL2561.cpp, find the getFullLuminosity function, and comment out the line reading disable(); and the 12 lines reading switch (_integration){ ... }

Another solution is, to have the pixels update every 20ms using a timer. You can have the timer, interrupt the main loop every 20ms and update the pixel values. 

(I can't really help you with timers, as I myself set the registers for my timers, and ISRs directly, which isn't for beginners. But there are probably libraries for it, to make it more user friendly.)