SPD-SX PRO: Can I connect two pads to a single TRIGGER IN jack?

Yes. On the SPD-SX PRO, you can connect two pads to a single TRIGGER IN jack using a Y-cable (stereo plug to two mono plugs).

Refer to "Configuring the Pads and the TRIGGER IN Jacks (PAD/TRIG IN)" in the SPD-SX PRO Reference Manual and set the Input Mode to "TRIGx2", then set the Trig Type for each connected pad.