First off, try it out in Project. Create a small dummy project and see if it works.
In my limited testing, it doesn't look to produce the results you are expecting. Setting an 80% threshold on the resource and a 50% threshold on the task, still leads to over-allocation on days that overlap. You are telling project that the resource can only work 80% maximum, then on the task they are only going to work 50%, which just extends the task by 2x. When a task overlaps with another task, then that resource works 50% on one task and 50% on another task, breaking the 80% rule. You can go 40% on a task, but then will get the opposite problem on tasks that don't overlap.
It's possible to take another spin on this though. I used to be heavy into percentage resource allocation as it seemed to illicit the feeling of a more concrete estimate. I could talk to my stakeholders with numbers, which made it seem like we have really thought things through. It was easy to setup, but managing the progress was a nightmare and I just ended up fudging the numbers anyways.
A couple years ago I switched to focus more on deadlines, rather than what percentage of time are you going to put towards this. So in your example, rather than complicating the matter with different percentages, ask the resource when they will be done the work. Let them figure how the work you need done slots in against other priorities. This does two things, creates accountability for the resource as it is purely developed by them and produces an objective result that can be tracked. Updating your project each week now becomes just a matter of moving the progress bar and updating end date if required. I do this now for all my projects and have found that this approach increased success.