Allowing authors to choose a primary and a secondary topic for their submission

We would like to give the authors of our conference the possibility to choose a primary and a secondary topic for their contributions, so that they can indicate the order of their preferences.

Is this possible with ConfTool?

You can use "Topic Groups" for that purpose. You can set a weighting factor for each topic group.
When you set a higher factor for one specific group, it will give topics in this topic group a higher significance.

1. Please go to:
Overview => Settings => Main Settings for Paper and Abstract Submission
... and activate the "Expert Settings" on the bottom of the page (cogwheel icon).
Under "Special Conditions for Paper Submission" set "Enable Topic Groups" to "Yes".

2. Now go back to "Settings" and go to the new page "Manage Topic Groups":
Overview => Settings => Manage Topics Groups
Again, please activate the "Expert Settings" on the bottom of the page (cogwheel icon).
Please create two topic groups called for instance "Primary Topic" and "Secondary Topic".
Via the option "Assignment Factor (Weight) for Review Assignments" you can define a weighting factor for each topic group (view image).

3. Then please go to:
Overview => Settings => Manage Topics
Create and assign your topics to either the "Primary Topic" or the "Secondary Topic" topic group.

4. Please note that you also have to enable topic groups for each submission type:
Overview => Settings => Manage Submission Types / Conference Tracks
Scroll down to the option "Topics" and set "Topic Groups" via the select box.

You can find more detailed information here:
Allow different kinds of topics for the categorization of submissions (Topic Groups)

A simpler approach would be to work with topics only and let authors add keywords for refining.
Please be aware that keywords cannot be used to match submissions and reviewers.

When you go to:
Overview => Settings => Manage Submission Types / Conference Tracks
... you can allow authors to select one or two topics for each submission and furthermore allow them to enter keywords.
Maybe that's already fine for you.

Another alternative would be to use submission types / tracks as primary selection and topics as secondary selection.

If that meets your requirements, please create the "main topics" as Submissions Types here:
Overview => Settings => Manage Submission Types / Conference Tracks

Then create the "secondary topics" as Topics here:
Overview => Settings => Manage Topics

However, reviewers can only select topics as their area of expertise. They cannot select a submission type / track.
Therefore, the automatic assignment process does not work correctly in this case, since only the topics are considered, but not the submission types / tracks.