You have it backwards OP. It is actually impossible not to level SCH and SMN at the same time. Both jobs belong to the game's only branch class: Arcanist. Gaining experience points with either job levels up your arcanist, and the two jobs share the level, stats, traits, and abilities from that class.

It is however, totally possible to gimp one of them. Stats is one thing, but so is gear. Once your item level gets to around 45-ish, the gear these two jobs use start to become more specific. They may share exp, but they do not share gear and arms in the upper levels, and it can be difficult and time consuming getting things like Ironworks gear for each job if it is your first time through.

As far as stat distribution goes, pump everything into MND if going SCH, and INT if going SMN. You can buy hymns from your Grand Company for 250 seals to redo your stats when you switch over to either job. 250 seals is crazy cheap, and can be obtained within minutes even if flat broke on seals.