Topic Quick question:
I wouldnt bother till 80-85+, even then you really wouldn't have to worry about having 2 sets until 90...and even then for raids, not heroics.
Till then while leveling, if you want to tank/dps just use their respective spec's. They can share gear at lower levels.
The bag space issue isn't really an issue. Just put your gear in a bag, it's not hard.
The biggest thing is just getting used to the rotation, and resource management of whatever your wanting to play and familiarizing yourself with the class and it's skills.