Yes, you can do what you described. You do not even need to start with a freebie number issued to you by Google; if you haven't already obtained a free number, then just start with porting one of the two lines, wait until it is fully working for both inbound and outbound calls, then port the other line in.
If you are porting in land lines, you must first port them to a mobile carrier, wait a week for the porting to be fully completed and broadcast out to all carriers, then initiate a port from the mobile carrier to GV. Do NOT use one of the third-party SIM sellers like H20, MintSIM, or Lycamobile. Use AT&T GoPhone or T-Mobile Prepaid.
Note that if you not have to pay an additional fee to keep two ported-in numbers, because you are paying two port-in fees. The extra fee is only required to keep a freebie Google Voice number.
After you port in two numbers, both numbers will be considered permanent (they will never be reclaimed by Google for lack of use). All inbound calls to either number will ring the same phone on the single SP1-4 slot to which the account is configured. All outbound calls will display the caller ID of the GV number you designate as "primary". You can flip-flop primary/secondary roles for the two numbers at any time, immediately, for free, as often as you wish.
Inbound calls to to either number will be handled the same way by GV: forward to the OBi and/or real forwarding numbers, and they'll receive the same voicemail greeting.