if its a short length it might be possible to drill the blue pipe , drill the smaller pipe , tie them together , go to the other end of blue pipe and pull them through so you havent got a joint - we do that when the blue pipe has been covered in sand as it slides fairly easy as long as no fittings underground - it does have potential of going horribly wrong though if something snaps underground - nice pond though - when you say the other pipe fits inside - how snug ? would it be able to vasaline it and push all the way through - you would have to keep greasing though and your reducing the flow - cant you just find a bigger pipe that goes over joint and clip it