Hi DexM,
In regards to lodging your application through ImmiAccount, you create or login to your ImmiAccount and complete - Migration to Australia by a Partner (300) first. You will receive a transaction reference number (TRN). Then, your fiance creates or logs in to her ImmiAccount and completes - Sponsorship for a Partner to Migrate to Australia (300). She will be prompted to enter in the TRN. The TRN will bring the two forms together online. I recommend that you and your fiance have all your supporting documents organised and ready to attach to your online application in an acceptable format and size prior to starting. It will make the online application process alot quicker and less painful.
Good luck with it all.
Regards,
Ben