Bring an acquired company with overlapping addresses onto your routers

Your company (CORP) has bought a smaller firm (ACQ). Both use 10.1.0.0/16 internally. In three weeks ACQ staff move onto two floors at HQ and into one branch, and the business wants them productive on day one: their own servers reachable from both locations, internet access, and your shared DNS server 10.200.0.53. Renumbering ACQ will take about six months.

HQ has a core layer 3 switch (CORE1) and an internet edge router (EDGE1). The branch router BR1 connects to CORE1 over a Metro Ethernet circuit that carries 802.1Q tags. Your manager asks for a design that is safe now and does not block the later merger.