ObesityHelp

I would definitely not do Spin and Crossfit on the same day!

I like to do Crossfit on days I swim or do a light run workout.

As for whether you want to spin twice a week or once a week, it depends on your goals. I try to do one long bike ride, one tempo ride (short & fast) and one speed workout (usually a spin class) per week at a minimum.

My goal is to get faster and stronger on the bike without sacrificing the swim and the run. If you are only biking, then you can bike more. Though a little speed work goes a long way. But if your goal is also to lose weight, then the extra spin class is a good way to do it.

I would do the following if the goal was primarily to improve biking:

2 strength-based workouts (Crossfit works for that)
1 speed workout (Spin works for that)
1 long bike ride
1-2 tempo rides

You can actually get more from doing 2 tempo rides a week than 2 long rides.