Personally, I intend to keep my powder dry and fire only when SRAM comes out with an eTap triple. Your problem as I see it is that your present granny is around 30T and that is probably already too high, unless I way miss my guess. You could help things out with a new cassette, and they are coming out with 12 - 42 11sp cassettes which is the kind of point spread I'd want for a tandem, if my small ring was 30T. Chances are the paperwork will tell you that your present 52 (or 53) big ring has to go away with the new FD limited to 14 or so teeth capacity. I would try it out first before I ditch it. If it doesn't work it might actually work out cheaper to just by a complete unit double. You will almost definitely have to replace your cassette if you do this.