Yes, normally you will be able to select seats during the booking process.

However I have had some (web site related) issues lately with selecting seats during the booking process on singaporeair.com. After completing the booking I called the local reservations office and they reserved the seats for me. They even allocated specific seats for me and my family on the SilkAir legs, although through the website you can only specify aisle / middle or window preference for SilkAir.

So in case it should not work - call reservations and they will do it for you!