I am not an employment lawyer (IANAEL) but I was under the impression work week had more to do with payroll period than calendar week. We switched from Sunday-Saturday at my first job to Monday-Sunday. I'm pretty sure I know of other companies who did the same.
Personally, I'd have these employees shift their work week and take off during Monday-Friday to compensate. I think if you're going to have multiple techs work a weird schedule, the best and fairest way to do so is sit down and talk to them. They may have input on what days would work best for their new effective "weekend", and you can reach a consensus.
I worked one project at said first job where we worked three 12 hour shifts and a four hour shift, from Sunday night til Wednesday night, then had the rest of the week off. I very occasionally was asked to run a service call if it was something only I could do, but mainly they respected our really non-standard week.