Can I use my receptacle multi-tap on both ends of an extension cord?
It really depends what you're powering

Three cell phone chargers, you're gonna be fine.  But a table saw and a dust collector, ain't gonna happen.  

The power-factor-corrected loads (i.e. Their VA not their watts) need to be added up.   They need to be within the capacity of the circuit (15A=1800VA intermittent, or 12A=1440VA continuous).   You should be fine if so.  

Otherwise expect burnt and damaged splitters, and/or a breaker trip, depending on luck.