My recall is that a 10 T election should be filed with the first return that the allocation is made. Your description tells me you want to deduct the interest as "investment interest", which has some additional limitations. I just kept a note as to how I did the allocation to remind me each year of what I did last  year. 

See this excellent post by another AllStar volunteer https://accountants-community.intuit.com/questions/1665854-where-does-the-10t-election-for-interest-...