Try as follows (works in bootstrap template):

    [fieldset name="someName" class="row-fluid"]
    [field class="span6" name="someFieldname1" type="text" /]
    [field class="span6" name="someFieldname2" type="text" /]

Explanation: In bootstrap the page is virually divided in 12 columns. The "span6" class ist 6 columns long; so span6 + span 6 adds up to 12; or span4+span2+span6. Each row is seated in a fieldset with a css class "row-fluid".