"Legacy" in the context of Google Voice only refers to the older of the two web-page user interfaces. The service is otherwise the same; they are just two ways to manage your configuration.
Now: there are two possibilities: either you have one Google account with two different inbound Google Voice phone numbers (the original number plus the one you ported in), or you ported the number into a new/different Google account.
Which of those two things do you have? If it is the first option, and the first number was also ported-in, or the first number was obtained from Google's pool of free numbers, and you paid a second $20 fee to make it permant, then you can receive inbound calls made to either number. They will take the same inbound call path you set up, and ring the one SPx configuration you authorized on the OBiTALK web portal. Outbound calls will show the caller ID of whichever of those two numbers you set as "primary" in Google Voice settings. If both GV numbers are permanent, you can flip-flop the primary vs. secondary role at any time, for free, by logging into your Google Voice settings page on a computer's web browser.
If you want to be able to make and receive calls from either number, with separate voicemail greetings and with messages and call history kept separate, then you need two different Google accounts; one for each of the two numbers. You'd then provision another SPx on the OBi to be authorized to use the other Google account.
You can transfer one of the numbers to another Google account.