By 1) only using 1 BCAN instantiation in your design instead of 2. And 2) by googling "Number of BSCANs OVERMAPPED", because sometimes finding a solution to a problem really is that simple. First couple of hits all explain it.
It takes me less time to simply figure out the likely google terms, then validate the first couple hits to see if that covers the solution than it would take to type (read: rehash) the solution. And probably also more time than you invested in searching a solution yourself. Either that or googling is not your strongest suit, in which case maybe
this will help.
Short version: seriously, read the first 2 hits for the first google. It really does explain it.