Gerbils are social animals and usually live happily together. However it looks like you have got two with an age difference. Try coralling the elder one in one section of the cage and then giving them seperate feeds. In time they should grow and get used to each other.
Btw I have 2 guinea pigs, which are also social animals and will typically chase each other around and play about over their food. However, there is no harm in it. They will not harm each other.
