We have two use years spread five months apart. Advantage: it allows us to go any time of year and avoid the cancellation/banking risk when going too close to end of use year -- by choosing contract year to use for a particular trip we don't have to worry about cancelling after the banking deadline has passed and losing points as a result because of cancelling too close to end of use year..
Otherwise, it creates no benefits different from having one use year. It creates one minor complication in that you cannot mix the two use years to get a reservation for a single night. For example if 40 points are needed for one night and you have 20 in one contract you want to use for it and 20 in the other, you can't do it without using up your one transfer per use year. In other words, when using both for a single trip we usually reserve some nights with one contract and other nights with the other and link the two. You,of course, have to follow two use years and obey two banking deadlines but that is not really much of an annoyance or complicated task.