Two forms using one submit button I want to merge 2 forms together (user register form and node registration form). Should I use multiform module to achieve this or create new form using form API?
Option 1:

Think of one form as Node Form with more fields that you can reuse on user entity as well.

Now on node save you have flexible node api to hook in and create user. As an example


hook_node_presave($node) 
hook_node_insert($node)
hook_node_update($node)


You can use any of the above, most suitable would be node_insert I think as you would like to create user only when new nodes is created. Inside the hook you will get the user submitted values inside $node object, all you have to do is create user programmatically from it.
Which basically is building a user object and mapping of fields and finally do a user_save() on your user object.

Here is an example of how you can create user programmatically


http://codekarate.com/blog/create-user-account-drupal-7-programmatically


Option 2:

Form does not need to be node form, you can have a custom form as well using form api, in that case you will not need any of the node hooks instead process would be just like user_save() you will do node_save(). So inside your custom form submit handler you will be creating a node(how to create node programmatically) and a user(how to create user programmatically)