Is it a financially wise move for an unmarried couple to plan to own 1 HDB each?

Depends on how many room you plan to get currently. If you plan for a unit that is sized for future family planning such as 4rm or 5rm,then you need not sell actually. When you both do get married, she can just move in and help you with the payment.

Your partner also need not plan for a HDB at 35, both of you can stay in 1 hdb first and eventually look at buying condo for investment?

Personally I think it's financially unwise or any young couple to own any property, no matter what your Asian parents tell you, let alone 2, unless you're:

a) already rich and have money to throw around

b) able to live in the property AND you live in a market where compound annual property value growth is likely to exceed 8%...

c) your mortgage and maintenance costs are more than 8% lower than your potential rental income AND you live in a market where rental housing is in high demand compared to availability

If you don't meet any of those criteria then you would be better off renting (one place!) and using your spare cash to invest in assets that will generate more wealth in the longterm - businesses.