When using mtline model in cadence spectre schematic simulation, why I
can not connect output of one mtline to output of another mtline?
(warnings show there will be short circuit and floating nodes)
It's because the symbol has the pins on the left as being input pins, and those on the right as being output pins.
It won't affect the simulation - it's just a spurious check because the pin directions are set on mtline. It would be better if mtline had inputOutput pins as each pin - then the problem wouldn't occur. So I'd suggest raising this with customer support.
You can go to Check->Rules and set "Shorted Output Pins" to "ignored" and then the warnings will go away.