Regarding 1: Consider a two proton J-coupled system. The states of the interest would be aa, ab, ba and bb. 1-quantum transitions are 1) aa->ab, 2) ba->bb, 3) aa->ba and 4) ab->bb. The presence of J-coupling modifies the energy level diagram in such a manner that the energy difference corresponding to the transition aa->ab is different from the energy difference corresponding to the transition ba->bb. This is the observable coupling/splitting of the .a->.b transition. However, there is only one 2-quantum transition in the system aa->bb. This would be true for any system with N-coupled protons, the N-quantum transition corresponds to the SINGLE transition between states a1a2...aN -> b1b2...bN and hence there is no observable 'splitting' (due to J-coupling) for this transition. Regarding 2, cannot say anything without knowledge of the contest. It may refer to the difficulty of converting N-quantum coherence to observable 1-quantum coherence, when N>2. Double quantum coherence can be converted to single quantum coherence with a single 90 pulse under favorable cases (e.g **I**kx.**I**ly -> -**I**kx.**I**lz by a 90x pulse).