Under Part 107, section 107.31 recognizes three roles: The remote pilot in command, the visual observer (if one is used), and the person manipulating the controls. The person manipulating the controls may be the same individual as the pilot in command, and the use of a visual observer is optional, so even though there are three roles, there might be one, two, or three individual persons involved. 107.31 insists that the people holding all three of those roles maintain VLOS throughout the entire flight.
So if your RPIC stays at the launch point, the drone can't legally be flown beyond the RPIC's line of sight, regardless of whether a person manipulating controls at another location can see the drone.
However, there is an interesting bit of wording in 107.19(a), which says "A remote pilot in command must be designated before or during the flight of the small unmanned aircraft." (emphasis mine). In my totally unreliable opinion, that seems to indicate that, during the flight, you may hand off the duties of RPIC from one individual to another, provided both individuals meet the requirements to act as RPIC. What else could that "or during" phrase mean? Surely it doesn't mean that you can take off without a pilot in command, and only designate someone as RPIC a few minutes before landing in order to make the flight legal! Note that there is no explicit requirement that the different pilots in command be co-located.
It seems that, if the two certificated pilots in command were able to effectively coordinate their communications, they should be able to hand off control and RPIC designation from one to another. Both would have to have VLOS during the handoff. But the person who is not designated RPIC wouldn't have to maintain VLOS during the time he's not playing any role in the flight. I may be wrong here, however.
I can't find any mention in the rules about multiple controllers, or which one might be designated primary or secondary, or anything like that. Those seem to be details that the rules don't directly address.