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the problem of the 1.5 launch architecture is largerly explained in this thread It's a little complex problem. Can we solve it together? >>>

however, I think that a single-launch archtecture is better than a 1.5 l.a. to avoid all (possible) missions' fail due to a "sum of delays" of the second launch (like happen, for months, at every Shuttle launch...)

but, also if the new rockets will be very reliable and never happen any delay, the advantages of an SLV or (best) the FAST-SLV are INCREDIBLE: a GIANT saving of time and R&D costs, less hardware costs, 50% more missions with the same funds, 4+ moon missions per year, etc.

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