I think the first thing you need to know about double bunk beds is how rare they are and how to buy them. If you want to buy full size stairway storage loft bed, I will tell you right away that you need to get it from a reliable company.
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If you want to buy this type of item, you will need to get it made or do some research online. Just googling those phrases doesn't help, as it's unlikely that anyone will do what you want unless you're in a really big city.
If money is not an issue and there is a good chance not to consider how expensive this unit is, buying to order is definitely an option. When connecting with a professional carpenter or cabinet maker, you need to point in the right direction.
Personally, I would recommend a carpenter as they are much better at handling wood in terms of detail. The first thing to do is ask your friend to recommend a carpenter if they can.
In protecting this person, you need to know that the people you hire know what they are doing. Then you need to send in your specifications for what you want and how big is the size with the measurements.
Sending a photo of a similar design is also useful because it can make you really want to grab the carpenter's eye. From here you can discuss and negotiate prices and how long it will take to install a double bed with a double bed.