Yes. You can do it all on Sydetasker! You need to have your credit card information entered into Sydetasker to post tasks. You can also offer on tasks if you have your bank account information entered & have your profile verified.
A single account allows you to post tasks as a Poster and make offers as a Tasker. If you're looking to switch between being a Poster and Tasker, you do not need to create a second account.