moonlighting
Recently I was talking with someone about moonlighting, and the question came up: what exactly is wrong with moonlighting, anyway? In trying to sort out my answer to that question, I've concluded that it's "it depends".
One issue is conflict of interest. If you're the CTO of Google and you pick up a job as lead programmer for an up-and-coming search-engine company on the side, I'd argue that you have a problem there. At the other end of the spectrum, if you're working shifts at both McDonalds and K-Mart, or even if you're the CTO of Google and you're also working at a local restaurant, who cares?
But quite aside from that are the questions of the type of work and your own abilities. Specifically, if you have a job that requires some sort of creative energy (Google yes, McDonalds no), then you have to ask if the second job is drawing effort you "owe" to the first. I'm not saying an employer owns you 24x7, of course, but if you're, say, a salaried lead programmer, you're probably thinking about architecture, algorithms, and your particular problem domain at times other than when you're billing your time. That's a good thing; personally, I have some of my best ideas either in the shower or while driving in to work. (And sometimes Shabbat afternoon, but if I find work thoughts popping up then I try to banish them.) So if you're a full-time programmer with another gig on the side, do you have enough creative juice to go around so that you're giving them both the level of effort that you would have otherwise given the one? For some people the answer is yes and for some it's no; you have to know yourself here. (And in some ways you can benefit from re-use; yes for architecture and no for specific domains.)
If you are the sort of person who can manage that, then there's still the issue of appearances. Often appearance is more important than reality in the professional world; if your peers or employers think you're shortchanging them, it's going to be a whole lot of hassle to convince them otherwise. So you have to decide if it's worth it.
I've been couching this in terms of employment, but it can apply in other areas too. The consequences are less severe in a volunteer or low-pay millieu; if I sing in a congregational choir and play dance music once a week for the SCA and play blues every Saturday night in a club (to choose three things I'm not currently doing), it may be that I'm spending less time rehearsing any one sort of music than I would otherwise, but so long as I'm meeting the minimum obligations no one's going to argue that I should be kicked out. On the other hand, if it appears that I'm shorting the dance band because I'm hoping my blues career will take off, that could engender bad feelings even if it's not true.
So if being a lay leader at two different congregations (in two different movements) is moonlighting -- a question I haven't actually addressed -- then is that a problem? On the one hand, I'm serving both competently. On the other hand, it's two different liturgies and maybe I'd know one of them better if the other weren't distracting me. On the third hand, it's a volunteer activity and not a major time sink. On the fourth hand, one could argue the appearance of a conflict of interest because I'm trying to do the same sort of thing for two organizations that wouldn't normally cooperate in that way.
I don't think it's a problem or I wouldn't be doing it, but I am also mindful of the appearance. And that's why I wanted to know if my rabbi sees a problem with this.

Re: Hmmm.
I agree that employers have a right to mandate results and no right to mandate means. (Unilaterally, I mean -- any employer-employee pair can put whatever they like into a voluntary contract.)
I think I'm hung up on the appearance aspect, sort of a combination of (if you'll excuse the domain switch) marit ayin and placing a stumbling block before the blind. Marit ayin is giving a false negative impression; in this case that I, and by extension possibly others, do not meet my obligations even though I do. Is it a real concern or hyper-sensitivity? Probably the latter. The other aspect is that if I do the balancing successfully my coworker might see only the results (hey, two jobs!) and not the effort behind it, go off and get another job himself, and shortchange our shared employer. The shared employer has ways to deal with it, but I've still led to that employer taking a temporary loss by being a bad role model. Is the problem ultimately caused by my coworker? Yes, of course. But did I contribute? Now obviously we can't go through life worrying about every little way that someone might imitate us and do it badly, so maybe it's not a major issue, but I have to ask myself the question.
On a vaguely related note, I was talking with some coworkers today and suggested that we find our peers within the organization -- employees of other recently-bought small companies -- and share coping strategies. My coworker pointed out that by all appearances we're getting more freedom and flexibility than the others did, and do we really want to bring that to our peers' attention? Maybe it could help, but more likely it would do harm.
I'm rambling. I'm sorry.
Is it conventional or legal for a widget buying company to insist that their widget vendor sign an exclusive contract to only vend to them?
Conventional? Probably not. Legal? Wal-Mart. And some producers require their distributors to agree to not carry the other guys' products -- Coke and Pepsi both do this a lot with restaurants. The restaurants, of course, are free to go elsewhere for their drinks, but (e.g.) Coke says "if you want us you don't get Pepsi" as a condition of the contract.