Can I connect 2 standard 12V 2A wall adapters in series?
Connecting two power supplies in series is possible, if they have isolated/floating output.

It means that they must have ungrounded 2-prong mains plugs to begin with, or in case of grounded 3-prong mains plug, the output must be said to be isolated/floating so that it is not internally connected to ground inside the power supply.

Devices with 3-prong grounded mains plug must have the ground connected; do not modify and remove the ground (earth) wire, it is not what you want, and it may be hazardous.