“When your girlfriend stays at your co-op apartment, she has no real “rights” to the apartment. She is just a guest. Under the proprietary lease, she is not even a family member entitled to occupy the apartment. In fact, if she were to stay there without you also being in the apartment, you might find that your co-op would complain about her occupancy.
“Many buildings require that the shareholder be staying in the apartment with the guest. There may also be restrictions on how long a guest can stay in the apartment. For example, if you were away on business and she was staying alone in the apartment for sixty days, they might consider her to be a subtenant and ask you to fill out the necessary paperwork and pay the sublet fees.
“When you talk about living in each other's homes if one of you should predecease, you are really mixing in the concept of "joint tenants" or "tenants with the right of survivorship." This involves having both of you on the stock certificate. If you are not both on the stock certificate, if you were to die, the heir(s) who inherit the shares to your apartment would be able to ask her to move. You could put her on the stock certificate, but you would then be giving her an interest in the shares to the apartment and the building might want to review her financials. before allowing the transfer.
“This is really a very complicated question and you should definitely consult with legal counsel. The issue for the house in Pennsylvania would involve Pennsylvania law and you would need to consult an attorney there. The other alternative is to get married. If you do that, then you could be tenants by the entirety and the living spouse would inherit the shares as a matter of law.”