I'm very ignorant in most of this (printing).
I do have a buddy that has a shop in town that has helped me a TON... he's my contract printer and is always showing me and encouraging me to just buy equipment and start making money. Then again... he's confident in his clientèle and isn't worried about losing the jobs. In that same respect I wouldn't go after his clients (knowingly) and try to steal them. But then again that is the type of people WE are. Your competitors could be completely different.
I also had my contract embroiderer basically walk me out of her shop because I was there to ask some questions about stabilizers for a particular garment I was trying to embroider. This was a client of a completely different business and they have about 20 single head machines like mine and two 6 head machines. I shouldn't be a threat in any regard. No matter... different type of person. Ironically, now I want to get big with embroidery and steal her clients.
I also make
http://www.cajunwineglass.com and sell them to her little boutique shop at wholesale. I'm not in anyway flexible with my pricing with her and the other day she asked for some LSU and UL etched ones and I did two samples. When I showed her she asked if they were gifts. I said, "Nope, these are samples" She ordered a dozen and I took my glasses and left. She gets NO special treatment from me if she is gonna be that way with her business.
I'm also in the computer business... I'm friends with several other repair shops and consultants. We can call on each other to help with jobs, problems or whatever. We are all very friendly and share tips on how to clean up stubborn new viruses all the time. I have often accidentally tried to steal clients but as soon as I find out who their IT guy is I tell them how good they are and that they are doing well by using them. We (the other IT guy and I) then laugh at the situation and sometimes he offers me the client because they just don't mesh well with them or something. He also refers clients to me all the time (he strictly does businesses now and I still do both residential and business).
But all that being said if a client says "hey, we are also gonna use Joe Blow's computer repair as well as you... could you show him how you do X,Y and Z?" I'd have to say no. I don't know Joe Blow and I'm not training them to steal my business. Will I lose that client anyway? Probably... but they are settling for inferior work now. Same as these companies that try to contract me out to go and do their jobs. They are large national IT support people that don't have a local office and just contract the job to the lowest bidder in the area. Well, you aren't going to get my level of service unless you call me. If they go with this big company and I do the work then they are getting MY level of service without growing MY company. If I DON'T do the job then they call some other company that probably won't do the job as well and maybe one day they will get fed up with that level of service. Probable? Probably not but I'm sticking to my guns here.
I'd say for your situation I'd just say "sorry, I don't have time to teach others how to do these jobs... especially if I'm going to have to try and keep up with the demand you are expecting. Stick with me and you will have this quality of work on your products consistently over and over."
Hey, if you really can't keep up with their demand they should just raise their prices... simple economics of supply vs demand.