If the result is good  and  the level of journals  is high, 
I would submit to different journals. There is no disadvantage to the reader (especially if you put the papers in arXive);  no silly question of the form "may be the author has good relations with somebody in the editorial body" will appear, and IMHO   the chances for acceptance are higher.  

Making two series of papers would require a lot of additional work (and that's why  I would not do it) but is also admissible .