How to link two buildings together ?

For disparate items that belong to a single site a site relation may be used but as stated on the wiki page, if there is a clearly defined boundary for the hotel and all buildings/facilities for it is is more common to either use a simple outline for the area with the relevant tags or a multi polygon relation.

If possible, putting tourism=hotel on a closed way surrounding the whole hotel complex might be the simplest solution.

• If the hotel is primarily defined by the buildings themselves, add them to a multipolygon relation and put the hotel tags (tourism=hotel, name=*, contact info, etc.) on that relation. If the buildings share a single address, put the address tags on the relation as well.
• If, in addition to the buildings, the hotel has grounds, gardens, pool, parking, etc, then better to surround the entire place with a polygon and add all the hotel tags to that. Adding area=yes will not be necessary in this case. And again, if the buildings share a single address, add it to the hotel polygon rather than to the individual buildings.

Avoid site relations; they're not well supported.

Site relations might sound good for this but keep in mind that they're only visible in the data. They are neither rendered nor found by OpenStreetMap's search tool (also see Usage of type=site relations question on this site).

So, if the hotel complex can't be put in a single closed way or the hotel consists of only disparate buildings, using a multipolygon relation is a good solution as it is better supported by many tools. How to add a relation using JOSM is explained here.