While I am not aware of any passage in the Numenera core rules that deals with this, I believe that's because of a misconception about how tasks and training work. It's impossible to be trained in the same task more than once, so there is no way to apply multiple skills.
Your character is attempting a task - such as identifying the plant creature (pg. 86 Core rules, Task Difficulty). If your character is trained in identifying plant creatures, reduce the difficulty by 1. If they are specialized in identifying plant creatures, reduce the difficulty by 2.
Two important things:
- Which skill to apply is based on the task being performed. There is a 1:1 relationship between these things. If a character is performing only one task, then there is only one thing to be trained or specialized in. If the character is performing multiple tasks, then there may be multiple rolls and skills.
- There is no interaction between skills. Being trained in two skills does not somehow mean being specialized in tasks that involve both.
The hypothetical character you mentioned is trained in both plants and animals. There is no interaction between those things. They are not specialized in plant/animal hybrid tasks. If the GM decides that being trained in plants or animals is sufficient to be considered trained in the identification of plant creatures, than the difficulty is reduced by 1. But there is no mechanism for considering two trained skills to be equivalent to a specialized skill.
The dash and jump example is more detailed, because running and jumping are different activities that each may incur their own chance of failure. If the character is merely running to get a start on a jump, then running is likely routine and requires no roll (pg.84 Core rules). The task is therefore only a jumping task.
A Different Example
A different situation occurs when the character is trained in one skill, but specialized in another skill that both seem to apply. For example, the player is attempting to identify the aforementioned plant creature. They are trained in tasks related to plants, but specialized in tasks related to animals. Does the GM lower the difficulty by 1 or 2?
In this case, the GM needs to determine which skill is applicable. If the creature is a plant, then the difficulty should be reduced by 2. On the other hand, this "plant creature" may not legitimately be a plant, only plant-like, in which case being trained in tasks related to animals may be more appropriate.
My Recommendations: Intrusions or Assets
Based on my experience as a Numenera storyteller, I recommend using an intrusion in these situations (if is interesting).
Player: I'd like to get a good look at the plant creature. Can I identify it? I'm trained in plant and animal tasks.
GM: Sure. Let's call it an intellect task using your plant training. The difficulty is 4.
Player: (Rolls. Does their stuff)
GM: As you are looking at the creature it starts moving erratically, rotating its branches is countervailing circles and swaying side to side. This is a GM intrusion, take 2 XP. Attempt a difficulty 4 animal-related task to anticipate it's movements and evade harm.
Player: (Rolls. Does player-stuff).
GM: Great! The plant-creature is a weird hybrid of plant and animal physiology. It appears to be hyper sensitive to changes in light. You were blocking its light, which it understood to be a challenge. It was prepared to attack, but you soothed it.
This has several advantages. First, it's a dynamic way to apply multiple challenges to a single interaction. It adds unexpected depth to what seemed straightforward. Second, it rewards players for having related skills rather than forcing which redundant skill to use.
If you don't want to introduce an intrusion, my other recommendation would be to consider whether having training in the second skill should count as an asset. Sometimes this is justified, but it can also be a slippery slope which invites your players to constantly ask for assets because of their skills. Use it sparingly and only when you think the situation really warrants it.
As an example, consider a locked security device which has something written on it in an old language. Using that device is clearly a numenera task, not a language task, yet understanding old languages would clearly be helpful. I may consider than a reasonable asset and decrease the difficulty accordingly.