2012 iMac RAM Upgrade
Solution 1:
Yes, you can and it's quite common practice.
- https://support.apple.com/en-us/HT201191
One concern might be how the memory channels are laid out, as it's generally better to have symmetrical RAM configurations to balance dual-channel access. (so you'd want each channel to be 4+8)
Solution 2:
This sure is possible and I have just installed a pair of 8 GB modules on my iMac a few minutes ago.
By not removing the existing two modules, the new cards just plug into the vacant slots - maintaining the dual channel configuration for a total RAM size of 24 GB.