Adding in any instrument means each person has to do less. Concept many dont understand which is why the big band concept is poorly executed with many solo musicians. Its kinda like thinking putting a bunch of solo superstars together automatically makes an invincible super team. If they cant adjust its nothing more than a big name mess.
The group i used to play with had an organist and a keyboardist, in addition to a 2nd keyboardist from time to time. And they'd all switch depending on the setting or song. This made it obvious of the roles because u saw that each approached organ, keyboard and 2nd keys differently.
If the song is keyboard led, the organ does what it does, usually swells and fills (but i've even heard some jazz style comping on the organ as well), but the 2nd keys has more versatility by choosing an accompanying sound thats appropriate (EP, Strings, Brass, Synth Pad, etc). Now depending on the song and style of the player depends on what you can do, because I've observed, there are different things that work, even within the SAME song. But it boils down to the concept of less is more. Both players can be going hard playing in every single space and doing runs at every single opportunity. Complement one another, as well as the other musicians in the group.
And if theres a competent bass player, by no means should any of them be doing bass runs!!!
