Work together and fixed duration

Hello,

In planning, we can choose many options for calculation, but there is some case where we need to combine some of them...

For exemple : we are working with external service provider (ie for development), then task is calculated as "fixed duration" because it's in then contract. But, sometime, when need some internal ressources too. If there is only one ressource, it works fine, but if we need more internal ressources, it can be difficult to plan exactly, because with fixed duration, ressources are tasked sequencely, so task may be calculated with the normal duration, but ressources are not planned to work together...

Is it possible to modify work together option so it can be fixed for all other planning mode (for example through a checkbox added right of the planning mode listbox) ?

Regards