Create your return as you did the 1st one and go into the 'edit return' option and put "IMG'return no. of the IMG created'".
For example , let us take that 2 IMG trains of nos. 45673 and 45674 have been created
Then go into the page of IMG45673 and click the 'edit return' button. Put IMG45674 as its return
You can also do...
more... the reverse.
Hope you understoos , bro.