The hardest packing problem is not two weeks of one climate, it is eight days that begin with frost and finish with a swim. Everything that serves the first half is heavy, and everything that serves the second half is easy to forget while you are still wearing a fleece. The method below starts from the overlap, since the items that work in both halves are what let you get away with a single bag.
Build the list from the overlap
Some garments earn their place in both climates. A merino long sleeve works under a shell in the mountains and on its own in a warm evening. A light down jacket that packs into its own pocket doubles as a pillow on the bus and disappears into the bag when the temperature climbs. Quick-dry trousers, a sun hat, sunglasses and a buff cross the same border without complaint.
The trick is to weight the bag towards the cold half, because warm-weather kit is thin and cheap to replace. Two T-shirts, a pair of light trousers and swim shorts take almost no space, so they can arrive late in the packing order. Everything bulky goes in first and everything adaptable gets squeezed around it.
Shoes decide whether the plan works. You cannot carry three pairs, so wear the boots to the airport and pack one flat pair that suits warm evenings. Sandals only make sense if they double as shower shoes at the places you will actually stay, otherwise they are dead weight for a week.
Choosing the bag
One bag is the goal, but not every bag suits a trip with two climates. A soft duffel absorbs odd shapes and squashes into a hostel locker, while a hard-shell carry-on protects a laptop and refuses to close once the fleece and the boots are inside. For a mixed trip we take a soft bag around forty litres and accept that it will be heavy on the first morning.
Wear the cold-weather outfit rather than packing it. Boots, the thickest trousers and the insulated jacket on your body removes a couple of kilograms before you start, and cabin allowances are checked more carefully in winter. Keep a foldable tote inside for the second half, when the boots come off and the bag suddenly gains a third of its volume back.
Split the load if two of you are travelling. One bag holds the shared items: the wash kit, the adapters, the first-aid pouch and the documents. The other holds the clothes, which means one person can deal with a bag search while the other keeps the passports in sight.
What to ship ahead
Sending a box is worth considering when the two halves of the trip are weeks apart rather than days. Postal and courier services handle this well inside a single country, and the common version of the trick is to post the winter gear home after the first leg, then collect it from your own hallway on the way back from the warm one.
Crossing borders changes the arithmetic. Customs rules, a forward address that only accepts one carrier and a delivery window that misses your departure can each turn a clever idea into a lost box, so use forwarding only when you know the receiving desk and the dates. Otherwise leave the item at the first hotel or in a station locker and travel light for the middle of the trip.
The cheaper shortcut is buying the warm-climate item where you land. A hat, a rash vest, a pair of light shoes or a bottle of sunscreen all cost less than the excess weight they save, and small towns sell them perfectly well. Do not apply that logic to medication, documents or anything you would have to cancel the trip without.
The changeover day
Plan the transition explicitly, because it is the day everything goes wrong in a packed bag. If the train arrives at ten and the next one leaves at four, those six hours are for swapping clothes, doing laundry and eating, not for sightseeing. Book a room for that night with a real check-in rather than a late arrival, and put the change of clothes somewhere you can reach without unpacking everything.
Layering helps in transit as well. Travelling from a cold valley to a warm coast means a train carriage at two temperatures, so the fleece, the shell and the base layer need to come off in that order and go back on in reverse. Keep the warm-weather shirt on underneath and you can change the world you arrive in within a minute of stepping onto the platform.
Winter boots are the one item with nowhere to go. Shoved into a locker they are fine for a few days; posted home they cost money and time. If you will not need them again before the flight, weigh the option of leaving them at the first hotel with a return date, which is common enough that most reception desks will say yes.
The cuts that make it fit
Cut the second pair of anything before you cut the first. One warm layer, one pair of trousers for the evening, one swimsuit. Books and guides go onto the phone, the towel becomes a microfibre, and the spare camera battery gets used rather than carried.
Then cut the categories nobody notices: the formal outfit, the second pair of trainers, the hairdryer that the accommodation already has. What remains should be a bag that closes without sitting on it, because the real test of packing for two climates is whether you can still lift it up into the overhead rack alone.