We will have 15 days in Australia. We have seen most of what Queensland, NSW, ACT, Victoria and Northern Territory have to offer on previous trips. On this trip we definately want to go back to Sydney. We would like to see WA and maybe Hobart. How should we devide our time between WA and Hobart? We plan on staying three days in Sydney. Thnaks in advance.

when?

We will be in Australia at the end of October and will leave in early November.

October is a beautiful month in Tasmania with Spring in full bloom so Tasmania would be a great trip for that reason alone, notwithstanding all the other reasons to go to Tasmania.
However if W.A was in the wild-flower season then that is really something to behold but I do think it is more September than October so anyone from W.A can put that right for me.
I think if you were to do the three areas you would be pushing it really. It takes over 4 hours to fly to Perth so by the time you add the extra wait times etc on that you have virtually lost a day. Tasmania by the way really needs more than a week to even scratch the surface.

Depends on what you want to see in both places - if you want to see more than perth and surrounds in WA, you may need mroe time there.

If I were you:
3 days sydney
5 days tassie
7 days WA (including basically a day to fly WA - sydney presuming that's where you leave from. if you leave from WA, then maybe 6+6.)

In late october, you could leave hobart at 7 in the morning, go via melbourne then be in perth by 10.25 with qantas, so that's not too much of a drama.

I think you could see all three, if that's what your aim is. Whereever you go you can spend longer, but bare bones you could do both. perhaps not try and see all tassie - stick to hobart, central /north east loop?

It's probably much hotter in WA than Tassie but the Kimberley region in north western WA is highly recommended. We took a scenic flight over the Bungle Bungle and it was an amazing experience.