[vivegi]

It really depends on what the terms of your contract with $day_job is. For example, if it requires you be on call and get to a physical site for troubleshooting and if your $construction_job would get in the way of that in the normal course of business (eg: you are 100 miles away when you get a support call), then you need to figure out how you can reduce scope with your $day_job so that you can do justice to both.

It is about setting expectations.

I am assuming your $day_job employer would prefer to have you work 20 hours instead of current 40 hours instead of the 40 hours going to 0 hours (i.e., all or nothing).