The true secret here is that it is not just the caster that is the cause of the problem, the spells themselves can create problems as well. While the first is well documented, the latter is not well known.
Witches from all over have tried to solve this problem already to varying degrees of success. They know that each of them have a slightly different connection to the orgone around them, which resonates slightly differently and causes interference. This is the nature of the magic that they wield.
Other answers go into the details of how witches can better harmonize each other's magic to better collaborate on larger spells. Or how to minimize the interference that multiple casters creates.
Another idea that builds on harmonizing is the concept of the coven or the ritual of the coven. The first ritual that establishes a group of individual witches to form a coven is designed to harmonize the magic of individuals better by sharing a small portion of their magic into each other member of the fledgling coven.
The net power gain of this ritual is zero -- that is intended. What it does is introduces the disharmony of another's connection into your orgone so that you can learn to resonate with it, thus lowering the interference when working together.
But how about a potential different approach to the problem: It is not the witches that are the sole problem here. Part of the problem is the spell, and it may be the true root cause of the issue.
When most people design and cast a spell for their use, it is intended to be cast with one person and one person's pool of orgone. But when you get multiple magical inputs into what should be one source, the multiple streams of orgone combined with the geometries of the spell and words of the ritual creates or enhances the interference of the individual parts.
Alternatively, a spell for a coven is designed with the coven in mind. A set number of witches collaborate on the spell, and the spell is designed to take these multiple inputs and construct one grand spell from them. The very basis the spell takes this teamwork into account, whether that is a group of equals, master and apprentice, or a simple number of taps to make a set amount of power.
This also explains why certain rituals work better (or worse) than the known magical theory suggests they should: The spell is being used as it was designed to -- not just in the area of power requirements, but also with the correct number of casters and what they are inputting into the spell.
While you can do things not as designed, the farther you are from the original parameters of the spell, the more interference and disharmony there is in the final magic. When the spell's feedback combined with the witch's disharmony reach a critical point, Bad Things happen.
While the spell cannot completely eliminate the resonant interference of multiple casters, the designed coven spell does take into account that there is interference from the different members and either accounts for it, or even uses it in certain places.
I do not know how much about spell crafting is known by your witches -- if they have to create their own spells as they gain experience, or if they have some sort of grimoire that has their secrets in it. I could see some of the below scenarios coming to pass:
- A legacy grimoire passed down may have the spells but not all the notes needed to master it as the intent was to have personal tutelage in magic. The grimoire is basically a magical cheat sheet.
- A powerful ancestor might have created a spell that their descendants can't cast alone so they improvised a solution to the problem
- A coven might have a grand spell in it but each member only has a fraction of the knowledge, the possibilities are interesting there. Only by bring the knowledge together can the spell be cast at its most efficient.