It is possible to run two reactors off of one pump but in this case it is best to have them running seporately.

The ROWAphos will need a much slower flow than the bio pellets so running from one to another means you will either have too much flow for the ROWAphos or too little for the bio pellets. If the pump is powerfull enough you can add a T piece it to create two seporate controllable feeds. However you will find that if one reactor begins to block the flow will be increased to the other so it is difficult to keep a stable flow rate between the two, this can result in issues.

If the pellet reactor is placed after the ROWAphos reactor the water supplied will be stripped of PO4, the bacteria that grows on the surface of the pellets require a background level of PO4 for cell growth and respiration. If there is no PO4 the bacteria will not grow and the biopellets will not reduce nitrate. If the output of the bio pellet reactor is run in to the ROWAphos reactor the media would soon block from the bacterial mulm produced by the bio pellets as they do there job. The effluent of the pellet reactor should ideally exit beside the protein skimmer intake so the protein skimmer can pull out dead bacteria from the system and the Nitrates tied up within that bacteria.