Each class has its own separate set of bonus attribute points, which apply to the Job(s) that stem from that class as well as the class itself. Leveling LNC/DRG to 50 gives you 30 points to put towards LNC/DRG. When you switch to MRD, you start at zero again. Level MRD/WAR to 50, and you'll get 30 new points to put toward MRD/WAR along the way. Switch back to LNC/DRG and your 30 LNC/DRG points will be as you left them, completely untouched by your LNC/DRG points, and vice versa.

Say at 50, your base stats (before bonuses) are 300 each. On LNC, you put all 30 into STR, and on MRD, you put all 30 into VIT. On LNC/DRG, you'll have 300 VIT and 330 STR. On MRD/WAR, you'll have 300 STR and 330 VIT.