Hi,
sure that can be done, we do that as well, the easiest way is to simply create two separate modules with the different text and display each on the different checkout pages. As Jerome pointed out, you can use a component like Advanced Module Manager for that. Then you can restrict a module to only appear on pages with a specific URL for example, so you can have a module appear only on the page "/task-step/step-1" or "/task-step/step-2" or something like that.
A second possibility which does not require a plugin, is to use the if($this->step== 1 ){ you mentioned and then load the module manually through PHP, which can be done by adding something along the lines:
$modules = JModuleHelper::getModules("YOUR_MODULE");
$attribs = array();
$attribs['style'] = 'xhtml';
foreach ($modules as $mod)
{
echo JModuleHelper::renderModule($mod, $attribs);
}
You can google about that to find more information. Hope that helps.