I have had this happen previously. Just tried it exactly as you have described and it worked fine!!!
When it happened before I remember just finding a workaround, but cant recall exactly how I came across the initial issue.
Maybe solidworks is just throwing an odd tantrum!
You're right. I hadn't noticed that. Probably is a bug. I'll report it to my VAR.
I don't know why your method didn't work, but the way I achieve the same thing is to right click and select midpoint of the first line, then left click to select the second line and add a midpoint relation. I have never had any problems with that method.
Thank you. I got the following from my VAR, which he got from the Knowledge Base (I guess I should have looked there first):
Why is it impossible to ctrl+select midpoints of two lines when trying to apply a coincident relation?
This is the expected functionality. It is not possible to ctrl+select the midpoints of two lines since there is no point to select at the midpoint. If the midpoint of one line is already selected, using Ctrl+Select again on another line will select the line itself.
To constrain two line’s midpoints together:
1) Right mouse button on the first line, choose ‘Select Midpoint’
2) Ctrl+Select the 2nd line (don’t try to select the midpoint – just select the 2nd line)
3) Nowapply a “midpoint” constraint.
The twoline’s midpoints are now constrained.
A good one to put on your list of Questions