I'm not positive, but I thought the Astrale 8 was designed as a rear-wheel mount which, if it is, should provide you with enough signal wire to reach the front wheel (high on the fork) or rear wheel (near the rear brake bridge). Running a cadence signal wire from the stoker's position to the captain's cranks is an easy (but ugly) run, thus running both the speed and cadence to the stoker's rear wheel / cranks can often times be the cosmetically better solution.
A spider is another name for the four or five "legs" that come off the crank arm to hold the chain rings. There are a number of ways to put the pick-up sensor and magnet on either the captain's right or left hand cranks, e.g., sensor on boom tube, down tube, or mounted to custom bracket attached to eccentric with magnet strapped to right crank arm, left crank arm, spider leg, or epoxied to the inside of the chainring. It all depends on how tidy you want it to look.
It's no problem having more than one wheel speed pick-up on a front wheel; however, keep in mind that the magnets work just like the wheel balancing weights your tire shop puts on your car's rims. One or two small magnets placed on spokes next to the valve around the stem usually off-set the inherent out of balance weight distribution of a stock rim (less rim material at valve hole / more material at rim seam). Larger magnets (like Polar's) can be overkill.