Per the prior discussion thread:

https://pdn.pega.com/community/product-support/question/openspan-runtimeconfigxml-more-one-deployment-file

You can set up your RuntimeConfig.xml as desired and not specify a start up project.  Then you can specify the startup project on the command line when starting OpenSpan.Runtime.exe.  You could then create additional shortcuts on the desktop, one for each solution the user needs to run.

OpenSpan.Runtime.exe project="C:\dir1\dir2\solution1.openspan"

*NOTE: you would not be creating a standalone EXE, but would be launching the Runtime and passing in the Package file.