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As Geoff said - your scopes don't have to be aligned, but you do have to have no differential flexure (think very, very slight bending) as you move two heavy scopes around.
I must admit that I go for Off Axis Guiding with a sensitive guide camera even though I have 3 scopes on my mount (1 piggy back, 1 side by side). Differential flexure and/or mirror shift made long duration, long focal length guiding impossible with either of the other two scopes, whereas with an OAG - but only if you have a sensitive enough guide camera (i.e. a colour Meade DSI I won't cut it in my opinion - a Mono DSI II Pro does)!
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