Why not 2 bikes....you noted a small living space perhaps a wall stand similar to
http://www.nashbar.com/bikes/Product...00_10000_17502
you noted your GF is sporty, she might prefer her own bike.
the downside to this solution is that you have to keep the bikes clean so they look good on the wall.
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Life is too short not to ride the best bike you have, as much as you can
(looking for Torpado Super light frame/fork or for Raleigh International frame fork 58cm)