The 2 couples are different, of course. Dorothea's not Gwendolen. If Gwendolen's spoilt, selfish, coquettish, shallow, ignorant, frivolous, Dorothea's intelligent, simple, thoughtful, serious and idealistic. In some ways, it's Celia that resembles Gwendolen more. However Dorothea, for all of her intelligence, is inexperienced, naive and unwise, and she doesn't have Celia's keen sight. Apparently she's so used to seeing big pictures (her dreams, plans, ambitions) that she fails to notice details, without knowing that it's the seemingly tiny, trivial things that reveal a lot about a person's character.
Casaubon isn't Grandcourt either. Take the scene when Mr Cadwallader asks Sir James "Why? what do you know against him?".
"Sir James paused. He did not usually find it easy to give his reasons: it seemed to him strange that people should not know them without being told, since he only felt what was reasonable. At last he said:Everyone's against the match, but why? Let's talk about Daniel Deronda 1st. Grandcourt from the beginning already shows his self-conceit and egotism in the way he dismisses everything as dull, and something calculating and threatening in the way he constantly creates pauses in his talk with Gwendolen. There are also references to some rumours about his character and past life. Casaubon? He's too old, dry, ugly..., people say. But that hardly matters. The narrator says:
'Now, Cadwallader, has he got any heart?'..."
"But at present this caution against a too hasty judgement interests me more in relation to Mr. Casaubon than to his young cousin. If to Dorothea Mr. Casaubon had been the mere occasion which had set alight the fine inflammable material of her youthful illusions, does it follow that he was fairly represented in the minds of those less impassioned personages who have hitherto delivered their judgements concerning him? I protest against any absolute conclusion, any prejudice derived from Mrs. Cadwallader's contempt for a neighbouring clergyman's alleged greatness of soul, or Sir James Chettam's poor opinion of his rival's legs,—from Mr. Brooke's failure to elicit a companion's ideas, or from Celia's criticism of a middle-aged scholar's personal appearance. I am not sure that the greatest man of his age, if ever that solitary superlative existed, could escape these unfavourable reflections of himself in various small mirrors; and even Milton, looking for his portrait in a spoon, must submit to have the facial angle of a bumpkin. Moreover, if Mr. Casaubon, speaking for himself, has rather a chilling rhetoric, it is not therefore certain that there is no good work or fine feeling in him. Did not an immortal physicist and interpreter of hieroglyphs write detestable verses? Has the theory of the solar system been advanced by graceful manners and conversational tact?"Then she says:
"Doubtless his lot is important in his own eyes; and the chief reason that we think he asks too large a place in our consideration must be our want of room for him, since we refer him to the Divine regard with perfect confidence; nay, it is even held sublime for our neighbour to expect the utmost there, however little he may have got from us. Mr. Casaubon, too, was the centre of his own world; if he was liable to think that others were providentially made for him, and especially to consider them in the light of their fitness for the author of a Key to all Mythologies, this trait is not quite alien to us, and, like the other mendicant hopes of mortals, claims some of our pity."He's self-centred, we conclude, but this passage's not written in a tone of strong condemnation. The signs are dropped here and there, in the 1st 10 chapters, but they're subtle: Dorothea notices him not caring much about her plans; Mr Brooke tells Mrs Cadwallader that Casaubon's concerned about nothing but Church questions; Casaubon doesn't feel much excitement for the wedding and "concluded that the poets had much exaggerated the force of masculine passion"; he writes in his proposal that she's "adapted to supply aid in graver labours and to cast a charm over vacant hours"; he wants Dorothea to have a companion whilst in Rome only because he "should feel more at liberty" and not in the least notices that she's hurt... These details are small, vague, subtle, masterfully laid out- the suggestion of the unhappy marriage isn't as clear as in Daniel Deronda, the reader, picking up these details, just gradually gets the impression.
Another sign that this marriage will go wrong is Dorothea's blind admiration and submissiveness. Gwendolen mistakenly thinks she can manage Grandcourt, but she does hesitate, and accepts mostly because of her poverty, whereas Dorothea eagerly throws herself into this situation, without thinking. More importantly, Gwendolen wants to control her husband or at least to have her ways, whereas Dorothea always looks up to her future husband, values his opinions above her own, throws away her independence, justifies his actions, blames herself, sees Casaubon as an intellectual, a teacher, a guide, a superior..., feels grateful to him for loving her and choosing her to be his wife... "She was not in the least teaching Mr. Casaubon to ask if he were good enough for her, but merely asking herself anxiously how she could be good enough for Mr. Casaubon."
Such a relationship simply cannot turn out well.