Technically with the right engineering you could connect 2 monohull
boats together. The biggest issue will be the large forces, vertical, horizontal, twisting etc. on the structure connecting the hulls.
Then you would need to decide whether to run two masts or eliminate the masts from the monos and make a mast
step on the connecting structure. The forces on the mast
will now have to be engineered to survive.
Then you have to decide if you are going to have a deck
house between the hulls or not. One of the attractions of the cruising catamaran is the large deckhouse/salon. This structure now has to be engineered and the additional forces have to be engineered into the structure that connects the hulls.
At the time of design, layup
and building all of this engineering is integrated and planned together. That allows the structure to be lighter than if everything is done after the fact as an "add on."
When you are done you will have a catamaran that is heavier than others for the LOA
with a deeper draft
unless you have done the work and engineering to redesign or remove the keels from the original hulls.
When you consider the cost of all this work and material you might be better off with a bigger mono or a purpose built cat.