The "or" operator is part of the XPath spec.  Your test would be done as
below.

Also note that the "=" and "eq" operators are not equivalent and it's a good
idea to get into the habit of using them correctly. 


<xsl:if test="myfield/field_is_available/data eq 'Yes' or
myfield/seconsfield_is_available/data eq 'Yes'">

    ...

</xsl:if>


Take care