Hi, my last post Multiple relationships between Tasks–is this possible?, caused a lot of reactions. all of them are about using dummy Task. Here is one of them:
“Insert a dummy milestone to carry part of the logic between the two tasks. I.e. “Task1” > SS > “Start Task1” (the dummy milestone) > SS+1 > Task2. And “Task1” > FF > “Task2.”
OK! Let’s do it with Microsoft Project 2016:
First, I will create Start to Start relationship between Task 1 and Dummy Task:
Now, I will create Start to Start relationship between Task 2 and Dummy Task, with 1 day lag:
as you can see, Task 2 will start 1 day after Task 1 will start.
Finally, I will create Finish to Finish relationship between Task 2 and Task 1:
Yeah! I’ve got what I wanted! Wait! Do I?.
Let’s say that for some reason Task 1 Should start at 31.07.2017. I will put it in Start Date field, and I will create Start no Earlier than constraint:
Everything looks perfect. Task 2 is still going to start one day after Task 1 will start. Now suppose that Task 1 for some reason will have 10 days duration.
as you can see, Task 2 will finish when Task 1 is to be finished, but it will not start one day after Task 1 is supposed to start.
So, once again, you CAN NOT have more than one relationship between two Tasks! 
Regards!
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About Nenad Trajkovski
Location: Zagreb, Croatia
Occupation: Project Manager
Interests: Project Management and MS PROJECT
He was born in Zagreb, Croatia at 1963. After completing his college he started working on projects in different business areas (banking, manufacturing, automotive industry, distribution, oil companies, etc.) developing and implementing ERP systems into different companies. He has got a lot of experience working with people in different business processes and also possesses great knowledge in information technologies and financial services. Today he works as a business consultant, adn Project Manager in PERPETUUM MOBILE d.o.o. Zagreb. He is a regular lecturer for Project Management in MS Innovation Center in Varaždin, Logosoft Sarajevo in Bosnia and Herzegovina and SEAVUS GROUP in Skopje Macedonia. He was named the best lecturer of WINDAYS 2008 while his lecture was also voted as the best. In addition, he was in TOP 10 lecturers at the MS SINERGY 2009 and MS VISIA 2009, 2010. Shares first place as the best lecturer of KulenDays 2009 and 2010 and PMI Conference 2009 in Zagreb. He is also a regular lecturer in the MS Community. He is a Certified Accountant and a PMP (Project Manager Professional) and a PMI-RMP (Risk Manager Professional), MCP, MCT, and Microsoft Certified Technical Specialist - Microsoft Project 2010. From 01.01.2012 awarded with MVP (Microsoft Most Valuable Proffesional - Microsoft Project)!
Nenad Trajkovski 
Hi, It is obvious that if two activities have to start on the same time and finish also on the same time then they have to have the same Duration so the relations can b true. If there is difference in the duration, that mean have to stop to wait for the other one. Hence, you can break the shorter acitivity into 2 sub-activities, the 1st sub to be linked with the 1st activity as S to S, and the second sub to be linked with the 1st activity as F to F. Â See attached. Regards
And what if the shorter activity became a longer, because project plan has changed? then you are in Problem again 🙂