In the past I was taught that: “In no case shall lines carrying pressure from two separate Pitot tubes be connected together” (see MIL-I-6115; § 3.2.2 Multiple Pitot tube Installation).
Now I find that in some ex-Partenavia (now Vulcanair) models (P68, AP68TP) there are the following configuration:
§ 2 Pitot tubes signals connected together with 1 IAS Indicator;
§ 2 Pitot tubes signals connected together with 2 IAS Indicator.
I never found anything on the subject in the various FAA’s Advisory circulars and/or certification rules (FAR 23/25, JAR 23/25, EASA CS 23/25).
Welcome to your comments on the subject, information on the flight behavior (Side slip) of the Pitot tubes connection and some literature that could help me in the right design of the flight instrument system.