Generally, the coat and pants are made by two separate tailors even if they work under the same roof.
However, if you send the work to two different shops you may have the problem of chasing down one to get the work done. See, e.g., http://www.styleforum.net/showthread.php?t=237582
By contracting-out the separate parts you are acting as a clothier with all of the attendant headaches, such as getting the work done, getting the work done RIGHT, getting the correct fit, and getting timely delivery.
One unsung task of the clothier is getting all the pieces made in a timely fashion. If you decide to contract-out the coat to Tailor A and the pants to Tailor B, then you will have to be the coordinator. You may have to chase down Tailor A or B for delivery as well as to rectify mistakes.
In my opinion it is better to have the suit made by one maker who will take responsibility for getting it done once, getting it done right, and getting it delivered.
I have a couple of friends who approached a well known pants maker. The pants maker was working "direct" with the customer and charged a low price to reflect that there was no retailer. The price was excellent, but there was one hitch. The pants were never delivered. The pants maker kept-on making promises and eventually ducked the customers. The pants maker finally returned the cloth after getting a lawyer's letter. If the customers had gone to the local retailer they would have paid more, but they would have gotten the pants. It seems that one of the jobs of the retailer is to coax the work from the maker.
My suggestion is to use Rubinacci for the whole deal. If you want more handwork, order it, and they should be happy to provide it along for an added charge. Incidentally, I have heard that Ambrosi makes Rubinacci's pants. Rubinacci may be able to coax the work out of Ambrosi faster than you could.