That will depend on exactly how the hardware operates in your particular system. Two pieces of hardware cannot operate using the same hardware IRQs and port addresses, but once Windows is in control, it can steer virtual IRQs and allocate virtual ports. If the Creative soundcard demands the default IBM PC resources, and the mainboard BIOS is programmed to disable the onboard sound if this happens, the Realtek sound will turn off and Windows will therefore not find it when it boots up.
Old soundcards used to have physical jumpers you could move to change the resource allocations, modern cards normally do not. In the BIOS setup there is possibly an operating mode selection for the sound, if it's set to Auto with a manual option, then the board usually turns the onboard chip off when a soundcard is fitted. If there's a manual option, when this is selected you can choose to set the onboard resources off the defaults, leaving them free to be claimed by the other soundcard. Windows may then find the Realtek chip at the new addresses, or it might be necessary to re-install the Realtek driver.
Alternatively, it might be possible to manually reassign resources in the Creative driver, so it's the one that leaves the default settings available, but I wouldn't expect Creative to be that helpful, and I think to do that the card would need its settings held in flash ram (i.e. be a reasonably advanced card) so it didn't clash with the onboard chip before Windows booted up. An Audigy may fall into this category.
Edited by Platypus, 04 November 2014 - 09:48 AM.