Second display with iMac?

It depends on which Mac you have.
It depends on which display you have.

Why don't you tell us what you have, and what display you're considering?

 

Yes it can.

 

"was wondering whether the other one could be used as a monitor out port."

You will need a USB-c to displayport adapter/cable IF the display has displayport.

MAKE SURE you get one that is known to work with 2017 iMacs.
Be aware that some work with 2016 model Macs, but DO NOT WORK with 2017/2018 Macs.
The only way to ascertain this is to read product descriptions carefully and do your "due diligence" by reading the user reviews as well.

If the display has HDMI, you would need:
1. A USB-c to HDMI 2.0 adapter (MUST be HDMI 2.0)
and
2. A HIGH-SPEED HDMI connecting cable (not all HDMI cables are equal).

 

I had an extra 24" to my iMac 27" for awhile...just because I had one, and for the fun of it.
If you need 2 desktops to be visual at the same time, it's good.

But I actually find it easier to swipe between desktops, and less crowded on the desk now.

 

I hooked up an Cintiq 13" Tablet that I basically use as a display, but it also comes in handy as a drawing tablet. I like the small size for it saves space on the desktop, plus it's a good monitor when I do web design/development work. I don't have to keep flipping screens on my main screen and also it gives a different perspective on my website(s) for not everyone is using a 27" Retina display. Yes it basically plug and play just as long as you by an adapter cable to plug into the correct thunderbolt port.

 

yes.. it just around 25 dollar connector usb-c to hdmi. I also attach earphone to my samsung 21 monitor so i don't have to tweak sound between internal imac sound and earphone.. Before it dam annoying.

 

You can run up to one 5K external or 2 4K externals. You can run a 4K monitor and an external drive off the same USB C port using the right hub.

Here's the Apple support link with more info:
https://support.apple.com/en-us/HT206587

Yes there are plenty of el-cheapo monitors but why not at least 27" 4K?

LG makes a number of HDMI 4Ks and two 27" 4K USB C compatible monitors ($treet price $350 for HDMI –$600 for USB C on Amazon) plus the 5K sold through Apple ($1,299). Here's the current LG lineup:
https://www.lg.com/us/4k-monitors

You can also run external monitors through the USB 3 ports using DisplayLink boxes (many brands around $50). If the adapter says it uses the free DisplayLink software, it has the right chip set. DisplayLink boxes do not work in OS 10.13.4–6 due to Apple but work fine in Mohave and 10.13.3 and earlier.