Can you use multiple specializations? Like can you use the Assassin Talent tree and then also the Survivalist Talent tree for a Bounty Hunter?

Yes. The cost for these are detailed in "Investing XP" step of character creation (Chapter 2).

You can have as many specializations as you want, as long as you pay the XP for them. You have to pay just to access the talents, and then pay for the talents as well.

I'm confused.. I read page 93 Acquiring new Specializations and I dont understand the EXP costs. I understand about getting new specializations but I'm confused about getting the talents. How does the EXP expenditure work for new talents? Is it the same as base?

Buying talents is the same regardless if its the first or tenth spec tree. Its just buying access to the tree that costs extra with each additional.

Example:

You have the Assassin tree at chargen, you can buy any talents in that tree at the listed cost. Later you want to buy the Survivalist tree. This will bring you to 2 specs, so the cost will be 20XP. You gain the bonus skills as career skills (letting you buy them more cheaply than non-career skills) and can now buy talents in the Survivalist tree at the listed cost.

Later again you want to buy the Scout tree. This will bring you to 3 specs, so the cost would be 30XP. However, the Scout tree is in a different career (Explorer) so there's a 10XP surcharge, so it costs 40XP. You gain the bonus skills as career skills and can now buy talents in the Scout tree at the listed cost.

Hope that helps.