I think it is better to prove it,
when you put two things in the same altitude , only when they have the same speed, can fall on the grownd in the same time,
so, we can prove it :
1)we know that the acceleration of the earf (gravity or g)is the same for every thing on it :
we know: V=at +V1 and here it is
V=gt+V1
so Va=Vb because gta +V1a=gtb + V1b
and V1 is 0 =>gta=gtb =>ta=tb
(a and b are names of two assumed things)