I recommended this article a few years back, and, since I'm aware if it isn't screaming "LOVE!" or "REVENGE" or "IT BRINGS BACK YOUR EX!" most of you will ignore it, heh. ;) However the information it contains could actually assist you in any of those three things (if you're having someone else do the work,) so I suggest you read it. Karma Zain does explain the reason why 2 or more workers on one problem can be a bad thing far better than I could. ;)
I've brought this up again because apparently one or two spellcasters (at least 2 different sites are the number of sites that advise this, that I have repeatedly gotten in my email box,) are telling anyone to hire whoever the heck you want - it can only help - and that's not my view on things, nor is it the view of basically any professional spellcaster I personally know and would recommend. Spellworkers who agree that it's OK for your client to hire anyone else on the problem while you're working on it, and that this is of no risk to your client or their case, by all means feel free to (intelligently) debate why you feel that is so in the comments section.
Clients with spellcasters who believe it's fine to hire anyone and everyone to stir the proverbial pot that is your issue being worked on, I'm sorry, but I can't take your opinion here. In 19 out of 20 cases (or an even larger percentage,) you hired someone to help you because you know too little about spellcasting to competently solve the problem yourself using spellcraft.
Before you think I'm being mean and judgmental, I did, in my earlier days, for a very brief while, allow people to hire more than one person if I was working their case. Why? You'll laugh, but I worried I was just being mean and judgmental to demand otherwise, (having successfully worked for friends with a group of friends - each of us doing our own part,) and also figured most of the time someone isn't doing the work anyhow, while I am. After I had 1 client's spellwork go wonky (she had 5 other casters, when I thought it was just one other, who I spoke with anyhow,) repeatedly (wonky being a kind word for "totally fucked up and insane manifestations,") and after another person who completely blew up their work listening and working with another spellworker (who I don't know personally, and who, so far as I can tell, must work everything ass-backwards to avoid religious conflicts with affecting free will,) I realized, very-quickly, I should rescind my earlier feelings on "sure whatever you're doing with anyone else is OK." I relay this information to you to let you know that some workers probably thought the same way I did, and it's a dumb and reckless way to think. I shudder that I let my naivety (I was new to pro-spellcasting, not at all new to spellcasting,) guide that choice for even a short time, but I'm big enough of a person to admit faulty thinking...especially if almost a decade old. ;) I only needed 2 experiences very early on to know a person who wants a stable of workers is a problematic client, and that even one more set of hands stirring the proverbial pot can just create a horrid outcome. So, because a brief stint as a twenty-something knob taught me not to risk being a knob, and I avoid knobbishness when possible (my "being a knob" issue has greatly lapsed since my mid-to-late 20's, haha,) and I personally would insist you let me clear out someone's spell work before casting for you, and insist you stop working with them to prevent future problem manifestations. ;)
That link should be necessary reading, folks! Get to it,