cross coverage

Hi masters Can one explain to me when should we sample the values for the cross items in a cover with items? when the cross-cover event emitted? only when the related items cover event emitted? is it legal having cross over item sampled over a different event?

e.g.

struct S:
   event e1
   event e2
   f1,f2 :bool
   cover(name: X1, expression : f1, event e1)
   cover(name: X2, expression : f2, event e1) 
   cover(name: cross, items : [X1, X2], event e2)

BTW, Can we support "event e1, e2"?