Can you add two dB numbers together?

Yes you can!

For instance, if you cascade two 3 dB RF attenuation pads the resulting total attenuation is 6 dB.

Use the following calculator to calculate the sum of two dB numbers

Note that while two dB numbers can be added or subtracted, the same is not true for dBm numbers. Here is a calculator to add or subtract dBm numbers.