I ended up bridging port 1 and 2 on my mikrotik.
then plugged in a second mikrotik to port two. it passed the pppoe through to the cpe with no routing or rules from the "main" mikrotik affecting my network.
Not sure about Openserve. But it's definitely the case on Vumatel - Their CPE has four ports, and I've been able to run two separate routers off it. Obviously you'd need a second account, and the ISP needs to request that a second port on the CPE is opened up to allow the MAC address of the second device.
I'd imagine it's a similar process with Openserve.