You don't even need two strands. A single strand and a BX GBIC (different wavelengths up and down over the same fibre) will see you good for 100Mbps, 1Gbps or 10Gbps depending on how much you want to spend on your optics. You can get 40km and 80km versions of all three as well.
Given the big cost in fibre is splicing and termination, halving the number of strands is going to make a big dint in the role out cost. There is a slight cost premium for BX GBIC's at the moment but I am sure if you made telco sized purchase you would narrow that gap substantially.