Help required - earliest of two predecessors needs to be the driver

That a second predecessor must be started makes the model unreliable, it is not difficult to imagine many scenarios where there will be a finish-start gap between predecessor activities. In such case successor will not start as soon as first activity is finished but will be further delayed until second activity starts. 

Using renewable resources Activity Production and Activity Consumption is another way to make a reliable model to meet the scheduling requirements.

In most cases a good staffing plan shall model how resources are made available or no longer available based on how some activities move as the schedule progress. In such case fixing resource availability by using calendar dates only is not a good idea. 

David,

your proposal to use renewable resource leveling will work only if there is one more condition: that both preceding activities shall start before succeeding activity.

In this case in addition to assigning dummy resource it is necessary to link both preceding activities with succeeding activity with SS dependencies.

But in this case succeeding activity may start only if one of preceding activities was finished and another was started.

Using renewable resources is unreliable for this purpose, under some conditions some predecessor activities will be delayed by the resource leveling, in other cases it can be worse.

Tom,

I managed to skip Rafeal demonstrating that Spider's arithmetic and functionality is breathtakingly superior to P6.  Like showing a motor car to someone who is struggling to harness his horses to a cart.

Anyway, back in the stone age.

You can use resource in P6 for all sorts of schedule manipulation, not just for resources.

in an empty project add three activities with a "Fixed duration and Unts/Time" duration type

no relationships

show the "Activity levelling priority" column, and make your "successor" activity low priority

Add a new resource definition whose availabilty is x

add the resource to each of the three activities with a units per time period of x/2

When you level the project, the low priority one is delayed.

As soon as one of the predecssors finish, there is enough availablility for the successor to start

If your software is capable of modeling consumable materials activity production and consumption then it can be modeled if using consumable resources (materials) leveling.

The following illustrates a model for activity 3 to start after either Activity 1 or Activity 2 starts but it can be set up to start after either Activity 1 or Activity 2 finishes by changing consumable resource creation profile to happen at the end of the activities 1 and 2.

• Do not hard link activities 1 and 2 as predecessors to activity 3.
• Model material X creation at beginning of activities 1 and 2.
• Model material X consumption at beginning of activity 3
• Level consumable resources.

Good Luck.

put a contraint on it then

Thomas,

Can I ask is the issue that the successor activity starts as soon as either of the predecessor finishes?

If so, there is a levelling solution.

Thom,

To recap, you have two predecessors (A and B) with a common successor (C), and you want (C) to start as soon as EITHER (A) OR (B) is finished.  Thus, if (A) finishes on day 5 and (B) finishes on day 10, you want (C) to start on day 6, ignoring the FS relationship from (B).

Under PDM/CPM, P6 will schedule (C) to start as soon as BOTH (A) AND (B) are finished (i.e. on day 11).  The EITHER/OR case is not allowed.  To get what you want, you would have to examine the schedule and manually remove whichever relationship(s) drive the successor to a date that is later than the date required by the first-satisfied relationship.  (I.e. you have to remove all the CPM "driving" relationship until there is only one left.)  Removing these relationships is likely to leave dangling logic, thereby breaking the schedule calculations.

To do this automatically would require dynamic-switching precedence relationships.  P6 and other major project scheduling tools don't have these, and most experienced users don't miss them.

use a lag or increase the duration of the one that you want to drive it