I doubt they were smart enough to even think about stray signals.
But at some point in the process
...(think of the pictures of them in the area, dropping the back packs, and walking away)...
they had to pause long enough to trigger the detonations.
why? while walking away they push the remote control. not that hard and no need for a 3rd person. The 2nd bomb went off 10 seconds after suspect #1 walked away