Connect 2 external monitors to a MacBook Air (Mid 2012) I need 2 external monitors but I am having difficulties to do it on my MacBook Air mid 2012. My monitors are 2 Ausus VG278Q.

I tried different docks. Latest one (which I believee was my best shot) is a Belkin Thunderbolt 2 Expresss Dock HD. 

Later I found out that the Mid 2012 don't support more than 1 external monitor at all. 
Is there any solution to that problem ?

I've seen this other question: Will the Macbook Air (Mid 2012) support dual monitors?

My understanding is that your monitors need to be have a thunderbolt 2 entry, so you can daisy chain then. Is my understanding correct? If that is the case mini DP entries are enough or it needs to be really Thunderbolt 2 ? This is already old technology and having to buy monitors to fit this requisite is quite limiting and very likely not future proof.

Any other solutions?
The only monitor that can daisy-chain the display connection is the Apple Thunderbolt Display. Your Asus monitors don't have Thunderbolt hubs or 'pass-through' connectivity.

However, I'm surprised that the Belkin Dock didn't work: assuming that it had a HDMI port and a TB2 pass-through, then that's exactly what it's for. 

There are TB2 to dual HDMI adaptors, which might work, if you can find one.
https://www.techpowerup.com/149981/zotac-announces-dual-hdmi-output-adaptors

If you really want to invest in 'future-proofing', you might be better served updating your Mac.