A character can help someone to complete a task, giving them advantage to an ability check, but it has to be something they could do by themselves, right?
Imagine a cleric and a fighter (Eldritch Knight) have a scroll of Sending, they can't cast it yet but it's on their spell lists (the Eldritch Knight's is the wizard's), so they could try a spell casting ability check. Can the fighter help the cleric, even if she casts with Intelligence and the cleric with Wisdom?
On page 200 of DMG under Spell Scroll it says:
A spell scroll bears the words of a single spell, written in a mystical cipher. If the spell is on your class's spell list, you can use an action to read the scroll and cast its spell without having to provide any of the spell's components. Otherwise, the spell is unintelligible.
If the spell is on your class's spell list but of a higher level than you can normally cast, you must make an ability check using your spellcasting ability to determine whether you cast it successfully. The DC equals 10 + the spell's level.
So we are dealing with an ability check to cast the spell from the scroll.
Now let's look at Working Together on page 175 of PHB:
Sometimes two or more characters team up to attempt a task. The character who's leading the effort - or the one with the highest ability modifier - can make an ability check with advantage, reflecting the help provided by the other characters. In combat this requires the Help action (see chapter 9).
A character can only provide help if the task is one that he or she could attempt alone. For example, trying to open a lock requires proficiency with thieves' tools, so a character who lacks that proficiency can't help another character in that task. Moreover, a character can help only when two or more individuals working together would actually be productive. Some tasks, such as threading a needle, are no easier with help.
From these rules, it is clear that the fighter is able to help the cleric casting the spell because he could attempt the check himself. The fact that they would use different abilities for the check doesn't matter from a rule perspective, as it is the same task regardless. However, whether or not the cleric gets advantage depends on the DM's ruling on whether or not the help is considered productive.
Personally, I would rule that in a out-of-combat situation the help could be taken advantage of (two persons discussing how to properly use a scroll), but in a combat situation I don't see how you could possibly provide useful help within the 6 seconds (1 round) it takes to invoke the spell scroll.
It could, in theory, be done. But there are two DM judgement hurdles built into the rules as written that you must clear before you can proceed.
The Eldritch Knight and the Cleric both have what it takes to cast the spell each on their own. The spell is on each of their spell lists, and that's all the spell scroll item requires for someone to be able to attempt to cast it.
But they are going to use different abilities to do so. Does that make it a different task? Picture it: the cleric is chanting the scroll's words as a prayer for a miracle; the Eldritch Knight is incanting the words as a weave of direct magic. Or maybe in your DM's world, clerical and arcane magic are both the same, just backed by different teachings?
Thus, this hurdle is whether the DM judges that the tasks are actually the same, which is a requirement for helping to work.
Even if it's the same task, the working together rules have one more requirement: that it be the sort of task that can benefit from help. Normally, casting a spell is not something that anyone can help you with, so a DM could easily say "no" here.
A more generous DM might rule otherwise for the purpose of scrolls, judging that the spellcasting ability check is to correctly interpret the too-high-level spell, and that this can benefit from help. (Of course, this gets back to the first hurdle: can an arcane-type caster help a priest correctly translate the spell's writing into the correct method of invoking the miracle? That does seem a stretch.)
Assuming that spellcasting method differences aren't an issue for using the scroll, then a DM might say that help could be given on the spellcasting ability check through the principle of "two heads are better than one." In this case, the help given would be in the form of understanding and preparing to cast the spell correctly, and the casting itself would be done solo after the check-with-help.
Thus, the second hurdle is whether the DM judges spellcasting from a scroll to be a kind of task that can even be done together.
Ask your DM. Because helping is, by RAW, only permitted on tasks that the DM judges can benefit from working together, this can only be known per-DM.
