First off, I completely agree with you that the best course of action would be to incorporate a sump!!

But the parameters that the OP is trying to work with was stated as, "
So what is the simplest way of connecting the two together? (Im not interesting in hard plumbing or drilling either tank)"(post #1). That alone creates a hugh limit on possibilities.
Also stated,
"Ideally, i would get another white stand so that it is not so low,..."(post #1)
"Well having the frag tank higher than the display is not really an option because the stand would be very tall and absurd, unfortunately."(post #9)
Maybe I missed it somewhere but I did not see where the frag tank couldn't be set up as "With the tank tops at the same level..."(post #40)
I thought the OP was on the right track in post #36, although this part is a concern, "I have more water in the tank than the suggested level , also i have a HOB skimmer that dumps even more water into the tank when everything is off, so the water does go over #1 actually." Why is this happening? I did make the incorrect assumption in post #43 that the water level would drop. But OP also states "I would say the tank would be able to accommodate for 6-10 additional litres with everything off." If set up properly, the syphons should break before that point.
I know, I know,;Sorry details,details details.
But let's get back to the best idea, incorporating a sump.....