Using internal flash limits you to one shutter activation at a time. What you're trying to do can't be done with internal flash. You'll need to move faster to get into the frame, ensure no-one blinks or use an external flash.
This also applies in CL and CH modes.
As reference Pg 87 in the MANUAL.
"Note that only one photograph will be taken when the flash fires, regardless of the number of exposures selected for Custom Setting c3 (Self-timer; Pg227)."