Second marriage is predicted if there is both beneficial and malefic influence on 7th house, its lord and Venus. There are hundreds of combinations that are used to explain second marriage in Vedic Astrology. All of them are centred around a simple logic that there is separational influence on the factors governing marriage in a kundali. While predicting second marriage, one should keep in mind that 1st marriage is possible: i.e., there definitely is some beneficial influence on the marriage factors in the kundali.
Separation, divorce and widowhood are predicted based on excess malefic influence on the marriage factors in a horoscope. Therefore, there must be influence of Sun/Saturn/Rahu on marriage factors so as to break the marriage. Combust or debilitated 7th lord or Venus is also not good for sustaining first marriage. 2nd marriage, which is predicted from a different house than 7th has a play in the horoscope to cause second marriage.
On of the important question is, which house governs second marriage functionally? I am still looking into this aspect. Many authors are of the opinion that it is the 9th house in a kundali denotes second marriage, with the logic being that 9th is 3rd (denoting younger sibling) from 7th. This is one way of looking at the problem because in olden days if wife used to pass away, families would marry the younger sibling to the person.
One of the astrologers I have great respect for says that 2nd house in a kundali indicates second marriage, 9th indicates third marriage and so on. The logic is not explained, but since I have faith and respect in her astrological teachings, I look at this one as well.
Assuming that we are already at the stage where one separation due to any of the above reasons has occurred and we are trying to predict the second marriage. Now, our focus should be on this house and we should be able to use all the principles of prediction here. If there are all beneficial influences on 2nd and 9th house in a kundali, then we should predict successful 2nd marriage. We should still look at the dasha periods of house lords and Rahu as well as transit of Jupiter and ascendant lord’s transit over trikonas from 7th house in rashi and navamsha.