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.....:cool: