Sub-events

Sub-events can be required or optional, but I don't believe there is a way to require 2 events.

I would add a ready-only Static HTML field with a bold note that says, "Be sure to select 2 events below!" And then make a report to run now and then that lists anyone who didn't follow that instruction and send them an email.

How do you add a static HTML field on the choose events page?

With the assumption you are talking about sub-events, since that was the context of the original thread, you would go to the Event Tag for the main event, then Edit Details / Registration / Fields then scroll down to Custom Fields. Add Custom Field, select Static HTML Content for the Field Type, and check Read-Only. The HTML Box is where to enter the message you want them to see. HTML can be used to modify the format.

However, if you're talking about a higher up "choose events page" I'm not sure and hope someone else can offer a suggestion because I have use my own custom version of that using the HTML From Stored Procedure module.