Can I put two hard drives inside "iMac Intel 21.5 EMC 2544"?

Well, you have a bit of a problem here.

If you bought a HD version only of this model Apple did not populate the logic board with the needed connector for adding in a SSD. You can see the location marked in Red in this photo from the teardown SSD connector MIA!

In addition this is a very difficult system to open and you need to completely remove the logic board to even get to the SSD connector. Here is the IFIXIT teardown to review for your self iMac 21.5" EMC 2544 teardown.

I would recommend you sell your current system and buy one of the older models if you want to have two HD's (or a SSD for the second drive) internally. They are easier to open and add memory as well.

One last point the newer model does have the SSD connector populated (per the IFIXIT teardown of the '13 model). It's still a bear to open and you still need to pull the logic board out. Even still I would still go with the older '11 model.