start with balls and make it fun,
if you think they are getting bored stop,
try again next day,give loads of praise when they do retrive.
i must say tho,you have got your work cut out with two dogs the same age.
as they distract each other.
so you might need to do a bit of one to one with them.keep at it.
when they get used to retriving the ball move on to a dummy,
after they get used to retriving the dummy get a rabbit skin
and either stitch or tie it round the dummy.
this works for me,
i hope this helps.
all the best.
lofti.