Hi
Use a timer to produce the pulse train (toggle a bit ).
Then move the toggled bit to one port pin.
In timer ISR control whether or not 100mS passed after start up. If passed then move the toggled bit to the second port pin. Or write a simple 'one shot state machin' in timer isr. At first state produce one pulse. After 100 mS. passed then change the state and produce both pulses.
Regards.