First of all, you'd need to use a vcsv file (not a spectre.in file, as I mentioned before). Then you're hitting the problem that because the signal names are repeated, it only loads one of them (you've effectively got multiple signals with the same name on the graph, and whilst they are named sig<0>, sig<1> etc on the graph, that information is not retained in the file. Even if you edit the name of the signal (double click on the signal and change its name) that is not saved into the VCSV file (this works OK in ViVA in IC615 though).
So, the solution is to edit the header of the VCSV file after saving, and change the names to be unique (don't use any non-alphanumeric characters in the names to be safe) - so if you see mysig, mysig change it to mysig1, mysig2 for example.
Then you can load both signals from the VCSV file.
BTW, it was really the IC5141 subversion I was interested in; the simulator version is irrelevant. However, the key information that was missing before is the fact that you're plotting the same signal more than once from different simulation runs. That's important...