How to get one random entry from among multiple sections
According to the docs, you can pass an array into the "section" parameter:

{% set entries = craft.entries.section(['quotes','didYouKnow']) %}


There are several ways to get a random entry, but the method you've chosen is a very good one, because it randomizes during the initial SQL call:

.order('RAND()')


Lastly, you can cut out some of the extra legwork from your original example by calling first instead of find:

{% set entry = craft.entries.section('mySection').first() %}


When you retrieve your entry using first, you'll get a single entry instead of an array of entries. You can skip the for loop entirely, since you're already dealing with a single entry.

So to put that all together...

{% set randomEntry = craft.entries.section(['quotes','didYouKnow']).order('RAND()').first() %}

&lt;h1&gt;{{ randomEntry.title }}&lt;/h1&gt;