Originally Posted by JavBlan
Suppose I have 3 scientific projects running at the same time with it's own tasks.
At certain point some lab tests should have been done at the same place alltogether (in order to reduce costs). They don't have a specific order from the start of the project but they do need to be done before reaching that point.
When they are done, that task is finished, for all of them, at the same time...
To me, in this problem, this looks like the most natural way to follow the project flow path.
Maybe I am missing something.
I understand the philosophy behind GTD, and I do believe one must do the best effort to keep that set of mind in order to accomplish. But I don't expect it to be strictly rigid nor perfect...
If I may offer my opinion. In the original question presented the issue seems to be a single task belonging to multiple projects. In my first understanding this could have been a task of calling someone and needing to speak about the details of project x, y, and z. The common context is the phone; and in a seemingly single action three tasks can be completed. Creating links (alias) to a single task to multiple projects x, y, and z would not solve this issue as potential consequences of skimming over specific project details could arise or confusion of overloading details in all similar tasks might contaminate project reference material. All which could affectively harm any actions performed during the phone call.
In your further clarification of the issue, the problem is not in having the individual tasks, but in the necessity of them being performed simultaneously to save on lab costs and meet budget constraints. This restriction is fundamentally different in its origin from the initial statement and therefore needs a completely different approach. You are correct; the issue requires a multi-dimension solution.
The three steps in finding a solution would be;
1) Review often. GTD specifically coordinates like actions together so everyones common resource of time can be effectively budgeted. It is in identifying with what needs to be adjusted within a project plan where one finds new solutions to such issues.
2) Data presentation. Learn how to present data through perspectives effectively. When I am making phone calls related to projects I do not need to see the shopping list my wife needs completed by next week. However, if I need to see a phone call cannot be completed until one or more tasks in an unrelated context or project are complete—that data becomes vital.
3) Use triggers, modules/templates, and scripts. Each of these could take a while to explain, but understand using varying tools and concepts in association with an OmniFocus workflow can be quite beneficial.
In your requirement of keeping cost down by performing similar lab tests congruently one might only need to place a proceeding tasks in each project to signal a similar tasks needs to be done and each is waiting for the conditions of the others in order to proceed. Once all the requirements across individual projects are met the lab test can be performed. This type of coordination is not necessarily instituted from the beginning of a project and requires proper review to make adequate adjustments.
Finally, if you have not heard of or read "Creating Flow with OmniFocus" by Kourosh Dini, I strongly suggest you purchase the book and take the time to digest it. The author deals with the tool OmniFocus, as David Allen dealt with the GTD philosophy. This book change my entire approach on how to use OmniFocus beyond making seemingly complex one dimensional checklists.