Τhen he left the synagogue and went to the house of a man named Titius Justus, a worshiper of God; his house adjoined (ἦν συνομοροῦσα) the synagogue.
It is a remarkable coincidence that Titius Justus's house happened to adjoin the synagogue .... or is it?
Well, I recently read Stephen Catto's interesting discussion of ancient synagogues here. Thanks, Mark Goodacre, for the link. Catto discusses the Stobi synagogue inscription:
[Claudius] Tiberius Polycharmus, also (called) Achyrios, the father of the synagogue at Stobi, having lived my whole life according to the (prescriptions of) Judaism, in fulfilment of a vow (have donated) the rooms to the holy place, and the triclinium, with the tetrastoa, out of my personal accounts without touching the sacred (funds) at all. All the right of all the upper (rooms of the building) and the ownership is to be held by me, Claudius Tiberius Polycharmus, and my heirs for all (our?) life. If someone wishes to make changes beyond my decisions, he shall give the Patriarch 250,000 denarii. For thus I have agreed. As for the upkeep of the roof tiles of the upper (rooms of the building), it will be done by me and my heirs.
Until I read Catto's discussion I had not realized that Polycharmus had donated rooms in his own house for use as a synagogue. Polycharmus retained the rooms on the upper floor as his house, while the rooms below became the synagogue. There seems to be a consensus that this is the scenario. Indeed, many believe that the majority of ancient synagogues were formed from domestic houses. So, after his benefaction, Polycharmus's house (the upper rooms) adjoined the synagogue (the ground floor). Sound familiar? This provides a good parallel for the case of Titius Justus and explains the apparent coincidence that his house was part of the same structure as the synagogue.
My suggestion, therefore, is that Titius Justus (or perhaps his forbears) had donated a part of his house for use as a synagogue. The remaining rooms continued to be his house. Thus, his house adjoined the synagogue. Then, after becoming a Christ-believer, he performed a similar benefaction for the church, offering the (rest of) his house as a 'synagogue' for Paul's use. I think this makes sense because someone who is able and willing to be a benefactor of the Jews is likely, after conversion, to provide a similar service for the church.
This is important because it adds another data point for the debate about the origin of synagogue buildings in the ancient world. It also brings Titius Justus into sharper focus and fits nicely with the view that he was Gaius and Stephanas (see my earlier posts here and here and here). Incidentally, the Titius Justus-Stephanas hypotheses came out of discussions with Stephen Carlson, who was the first to connect the two names. I don't know what position, if any, he holds on the hypothesis.
Also incidentally, John Chrysostom wrote about Gaius:
See what a crown (στέφανον) he has framed for him by bearing witness to such great hospitality in him, and brought in the entire Church into this man's house!
Benefactors were commonly given a crown, and for Chrysostom, Paul give Gaius a 'crown' by acknowledging his benefaction. Chrysostom seems to have been a hair's width away from realizing that Paul had given Gaius a 'crown' (στέφανος) by naming him "Stephanas" (Στέφανᾶς).