Same reservation won't matter. But, same ticket will. As I read it, OP is booking either IAD-KEF-DTW on one ticket or IAD-KEF and KEF-DTW on another ticket. Either way, if he misconnects at BOS, it will be for FI as the late delivering carrier (presuming that FI is late) to rebook. OP would be rebooked on the next available B6 flight and possibly another carrier if there is nothing to be had on B6.

The disadvantage of two one-way tickets is that if any changes are made, OP will pay two change fees instead of one. Thus, if he needs to move his trip by a week, he will pay a fee to change each ticket if he has booked the two one-ways.