I have a macbook air from 2014. I connect one monitor to it through thunderbolt to HDMI connector. If it's possible, how could I connect 2nd monitor as well? (it doesn't seem to have mini-DisplayPort only 2 other usb ports.
You have to use a USB to VGA/HDMI adopter. You can grab one under $10 from eBay.
It will work but you might experience some heat generating at times. Also the monitor that you connect via USB can feel slightly laggy.
What I did was using the USB connected monitor for minor stuff which will require less refresh rate, more like references, social apps, etc and use the Thunderbolt one as primary.
Also, if you are planning to use this setup for long, try upgrading for a Macbook Pro. It has better cooling and has more ports. You will not regret the upgrade ;-)
This link from EveryMac.com will help you: