In reply to Nathaniel Gjöderum-Larsson:
> My girlfriend owns a 9.4cm (60m) single and I'm thinking of buying another single the same size. Would we be able to use these two ropes at the same time as doubles when climbing trad?
Yes but it's very heavy.
Personally I'd suggest either a 60m half rope if your interest is trad (fold it for 30m pitches, combine with the full for longer pitches if you really must) or a 70m lightweight full if your focus is sport. Or save the money for something nice, keep using your girlfriend's rope.
jk