There is an interpretation taht there are two Raptures....one prior to the start of the Tribulation, and the other prior to the start of the second half of the Tribulation.  

There is only one Rapture.  The Rapture of the Bride.  This is the only time when living people are removed from Earth and changed into glorified bodies.  And this is prior to the start of the Tribulation.  This is when the Bride, in Her entitety (resurrected and raptured Christians), is taken by the Groom to His Father's House.  No part of Her is left behind. 

Then, in the midst of the Tribulation, there is a resurrection, not a Rapture.  This is the resurrection of Tribulation Saints.....those who gave there life for their belief in Christ during the tribulation period.  These are not parts of the Bride that were left behind.  They are the unsaved who, after the Rapture, become believers and are martyred.  And they are an entirely different group than the Bride.  And as mentioned, they are not Raptured, but resurrected from the dead.