I asked a few days ago if there was any limitation on the location of township or city offices. Someone replied with a quotation regarding the township supervisor's meeting, to the effect that they could be anywhere the supervisors might choose.
That doesn't directly address the location of offices. Poking around on-line, I found, in 1933 Act 69 Second Class Township Code:
Section 1701. Township Buildings.--(a) The board of supervisors may procure by purchase, gift, devise or the exercise of eminent domain a lot or lots of ground located within the township and erect or use buildings thereon for township purposes.
2015 Act 67 (Third Class City Code) says:
§ 13601. Public buildings generally.
(a) Authority of city.--With regard to a public building, a city may, by ordinance, do any of the following:
(1) erect, purchase, establish or maintain the public buildings; or
(2) purchase, take, use, occupy or acquire, by any lawful means, including eminent domain, private land, buildings and property in order to erect, establish or maintain a public building.
So, it looks to me that the township is bound to erect any building somewhere within the township. The city has no such restriction. Hence, if a joint office building were to be erected, looks to me that it would have to be built somewhere in the township.