Both instructions must be simple . Shifts or rotates can only pair in the U pipe. ADC and SBB can only pair in the U pipe. JMP, CALL and Jcc can only pair in the V pipe. Neither instruction can contain BOTH a displacement and an immediate operand.
Prefixed instructions can only pair in the U pipe.
This includes extended instructions that start with 0Fh except for the special case of the 16-bit conditional jumps of the 386 and above. The U pipe instruction must be only 1 byte in length or it will not pair until the second time it executes from the cache. The flags register exception allows an ALU instruction to be paired with a Jcc even though the ALU instruction writes the flags and Jcc reads the flags.