A towbar-mounted bike carrier is really meant for use when solo, not when towing.
Don't forget that the weight of the carrier and the bikes, when added to the caravan noseweight, STILL has to be less than the noseweight limit of the car or towbar, whichever is the lower.
This may not give a problem on large 4x4s with 150kg noseweight limits but SUVs and cars need very careful consideration.