I have this situation configured but it doesnt works fine, maybe this is a kind of incompatibility between plugins:
I use "Parameterized Trigger" + "Run condition" + "Conditional build step" plugins to set a diamond execution but making the number of build in 2nd step variable (even 0) depending on the input parameters.
-One problem is that with 0 value "Counter parameter factory" crash (Java exception) so I use conditional build step to check it. By now that is working as intended but it would be nice Counter parameter factory not to crash with 0 repetitions.
-The main problem is when I launch another build it finish its main step but when it should continue to the 2nd step of the diamond it keeps waiting the downstreams builds of the other execution avoiding users to run more than 1 project at a time.
You can do this already - just specify a list of jobs to build in one step.