Brave Brown-Haired Belles Boldly Bolster Boyscouts

Linda Danvers was the product of a genetic experiment, spawned by an adversary of the Earth-One Supergirl in order to make six miniaturized clones of the heroine. After battling them, Supergirl used Gold Kryptonite to drain them of their superhuman abilities, and kept them in stasis. However, the mental bond that existed between them allowed them to merge into one unified individual. 

Years earlier, a brilliant yet wicked young woman what also resembled Supergirl resided in the bottled city of Kandor. This lass, Lesla Lar, had been turned into an energy being by fellow Kryptonian criminals who betrayed her. While Lesla, in this form, was finally destroyed after her failed attempt to take over Supergirl's body... it is quite possible that her mental abilities drew the tiny clones together. 

Using these mental attributes, they made Kara Zor-El forget about her civilian identity as Linda... only recalling her Supergirl alias. Eventually, she discovered the truth and confronted the clone, who assumed the brunette Linda identity. Rather than imprison her, Supergirl set up this second Linda with a new life and identity.

Susie Tompkins was the imaginative niece of Lois Lane of Earth-Two, and while under her aunt's care during various occasions she would come up with elaborate scenarios that she claimed to have witnessed or to have been a part in. This ultimately led to Lois' coworker and later husband Clark Kent, and his alter ego Superman, into getting involved in entangling the truth behind her fabrications. On one occasion, Susie even drew the attention of the 5th dimensional imp and foe of Superman, Mister Mxyztplk, who challenged her to a competition.

However, years later Susie's mental abilities would aid in the rescue of invisible aliens trapped in a Metropolis lake. These telepathic abilities mirrored that the second Linda Danvers. Could such mental attributes have mentally connected Susie, across space, with the mind of the comatose future Power Girl Kara Zor-L on her way from Krypton-Two to Earth-Two?

Interestingly, during their second meeting (although it was the first published) while Clark Kent was babysitting Susie, suddenly he was made to fall asleep! And, as it is conveyed, his mind is pulled into a "weird phantasmagoria" where he dreams about Cinderella while attempting to read this fairytale to Susie. As for Susie, she remains awake the entire time! During this dream, Clark assumed his Superman identity, and helps a Cinderella... also known as Cinder-Maid, to find true love with her fabled Prince George in this “dreamland”! 

This blonde young woman bares a striking resemblance to his long lost cousin Kara, who he would meet decades later when she becomes the Maid of Steel! And, in fact, Power Girl at that precise moment was also in a “dreamworld”, created by her father Zor-L aboard her Symbioship rocketing through space. Within this virtual reality, the hyper-extended life of Kara Zor-L ultimately housed a second persona within her mind, the Symbioship’s version of an idealized Kara which became the counterpart of Lesla Lar. Like Lar, she ceased to exist once Kara regained her mental fortitude as Karen Starr.

As for how Clark was pulled into a dreamworld where Kara was in... it is quite likely that Susie was the mental conduit through which this first ever meeting between cousins Kal-L and Kara Zor-L occurred, although neither Superman nor Power Girl realized it!

On Earth-S, Mary Broomfield was the brunette twin sister of Billy Batson, and she had a seemingly separate persona in Mary Marvel... sister of Captain Marvel. Just like there being a difference between Kara Supergirl and her cloned sister Linda Danvers… and a difference between Kara Power Girl and her second cousin-in-law Susie Tompkins… so too was the separation between the two Marys, Marvel and Broomfield.  

And thus, on all three Earths, the greatest heroes of those worlds had not one but two great gals who aided each other in their adventures.