Hi, from my experience almost all holiday airlines have a Max limit of 30-32kg per case and this would be classed as "Heavy". This is due to lifting restrictions for baggage handling staff(who have to be able to safely lift each case without breaking their back!). So combining luggage allowances (eg 2 x 20kg) into one case or having a case over 30 kg is really unwise, as at check-in they could refuse to take it and you've then got to unpack, repack OR ditch some stuff. On a recent flight check-in going to Palma via T.Cook airlines, a family in front of us had to do just that, with two young kids and a MASSIVE suitcase, they had to re-pack in front of the whole check-in Q, I felt really sorry for them, but couldn't believe they thought that a case weighing 40+ kg would be ok, the husband couldn't even lift it on/off the scales!
Check your tickets for each airlines limits, but I would not want to put all our eggs in one basket anyway, we regularly split our stuff over the two cases, so theres at least some clothes etc for each of us (2). Sounds like a chore but isn't any more difficult than seperate packing, and well worth it if one case goes astray, which again happened on a Palma flight to a couple who had to wait 48 hours for the missing case to turn and be delivered.
Ray