When one or more of your extensions are in conflict, they will not function as intended or will not function at all often producing just a blank page.
The easiest way to identify which extensions are in conflict is to use an extension conflict tool. You can find these by searching on Google.
Once you have found out which extensions are conflicting you will need to change the conflicting file of one extension (lets call it A) to inherit from the other extension (lets call it B).
class A_Extension_Model_Type_Onepage extends Mage_Checkout_Model_Type_Onepage
class A_Extension_Model_Type_Onepage extends B_Extension_Model_Type_Onepage
Once you have done this you need to change extension B config.xml file commenting out the conflicting
<rewrite> blocks that are now inheriting from A.
You will also need to identify any conflicting phtml files and merge the files into one phtml file incorporating all of the functionality of the multiple files. This means you will also have to edit any related layout xml files to point to the new merged phtml file.