You can, I'm an existing Globe Postpaid subscriber and my previous plan (PLAN 499) will expire on November but I added additional line (PLAN 999) which will arrive on the next few days. So I have 2 lines active now.
Each line is independent of each other. What you can do:
1. You can already apply for as many lines as you want, with or without any existing plans
2. When your contract is finished, you can downgrade, upgrade or even terminate the said plan without prejudice to your other existing lines.
As due diligence, Globe would just check if you would have the financial capability to support the additional lines so be prepared to bring supporting papers.