Abhinav entered the show with his wife. As compared to Rubina, Abhinav has very little popularity and a smaller fandom. Also keeping the entire game in mind, it has been very evident that these 2 haven't come int the nominations together, if they have then only once or twice. So when 1 is in nominations that fandom of the 2nd one votes for that person. Hence like this they survived in the game so far. Now if both of them come in the finale together, they would be a split in fandoms and in the votings, owing to which maybe Rubina might receive lesser votes which could prove to be an advantage to other really strong contestants like Aly, Rahul and Eijaz. Also as there are many rumours doing rounds that Rubina is the scripted winner of the show, they would want her to win organically. So she has to get maximum votes. Applying a mid week eviction like the one happened, Abhinav was sent only to increase the vote back of Rubina.
Ps. This is my opinion and observation of the game. Not meant to offend supporters of any other contestant. Personally even I am not a supporter of Rubina or Abhinav.