Hi Raul,
If I understand your question correctly, you are sure you will always have one record after random sampling to 50-50 .
Split works like mutually exclusive unless you select "enable overlapping" option.
Hence, whichever is the first sub-set defined would have the records as per the random sampling.
In your case, if you are attaching delivery A to the subset ,"Enviar Promo Millas", then the record would get this delivery.
If you still want to send both the deliveries to that one record , you can use fork activity after the split sub-set and attach both A and B deliveries to it so that the same record would receive 2 deliveries.

Hope this helps.
Regards,
Anita