Visit visa/partner visa

You can apply for 2 visas at the same time.

You can apply when your in Australia assuming you are on a visa without a no further stay condition.

You need to ensure your initial visa is being applied for for a genuine reason. Visitor visas are not granted to come on shore and apply for another visa, they are granted to come and visit Australia.

If you are onshore on a valid visa and apply for another visa you will be granted a bridging visa that will come into effect when the first visa expires.

If you are onshore and you are already on a visa without a no further stay condition then nothing is preventing you from applying for a partner visa now, you have no requirement for a visitor visa.

If you apply for a partner visa offshore then you can use a visitors visa to visit Australia whilst it processes but you won't be eligible for a bridging visa.

If you come to Australia on a visitor visa to see the sights or visit friends or relatives, without a no further stay condition and then subsequently decide you like it here and decide to apply for a partner visa then that would be fine, you would be granted a bridging visa.

You don't.

Some visas it is much more likely to be added to.

It comes down to the case officers discretion your visa history and a million other factors.

The easiest way to find out is to apply.