From the same blue post that gave the examples Allison quoted:

The Druid will select a raid or party member and cast Symbiosis on them.
The target cannot be another Druid.
The target gains a buff that tells them what spell they have temporarily learned, and that spell temporarily appears in their spellbook, and they can drag it to an action bar.
The Druid’s Symbiosis button will become the ability they gained from the target class.
The Symbiosis link is cancelled if: the Druid clicks off the Symbiosis buff, or the Druid or the target changes continents or enters or exits an instance, or the Druid or the target changes talent specializations.
Symbiosis fails to cast if Symbiosis is already active on that target.
Symbiosis cannot be cast in combat.
For the non-Druid target, the icon will revert to a placeholder (uncastable) Symbiosis icon when Symbiosis ends, but will become the Symbiosis-granted ability again the next time Symbiosis is cast on that target.
The ability gained from Symbiosis is always something chosen to match the Druid’s specialization and the target’s combat role.
The target always gets the same spell unless they change combat roles. A Druid of a given specialization always gets the same spell from each different class. If there is a change of specs, there is a change of spell.

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In other words, you get 1 Symbiosis per fight. No per-phase switching, or switching if your hapless symbiote gets themselves killed.