You can't do that now with the SourceSafe action itself (this will be supported in all Run Program-derived actions in the next release), but you could use a Run Program action calling
SSHELPER, mark the SSHELPER steps to not wait for completion, and use Wait steps to wait on each process to complete (see attached sample).
Another option would be to use two separate projects (or the same project with parameterized values for branch name, etc.) for branch 1 and 2 (if you want to get, build, etc. all in parallel) and launch them in parallel (see the end of the
Chain.bld sample). This way would also allow you to use the SourceSafe action directly.