If you've placed two C4 charges and use the detonator only the last charge placed will detonate. Then you automatically switch from the detonator to back your primary weapon. So need to select detonator again to detonate the remaining C4.

Left click/Right click?Middle mouse didn't seem to have an effect.

Would be nice if you could Left click to detonate charge 1 and right click for charge 2 or middle mouse button to detonate all. And not auto switch back to primary until both charges were used.