With UltraSonic or HAL-O Damping Instruments, one damper is all you need, reducing audible microphonics efficiently enough that a second usually adds little or no audible effect. With extremely microphonic tubes and with some huge tubes like 8.5" tall 805 tubes, however, a second damping instrument can be beneficial.
Every system and component is different, however, and personal tastes vary, so sometimes a second damper is indeed preferable. That's a matter of self-experimentation, however, and not a default or mainstream recommendation.
(With less-efficient dampers like silicone o-rings, you would sometimes need two or three dampers to tame an unruly tube. But then, with each additional o-ring, you suffer from more of the sonic trade-offs incurred with o-rings and further restrict convective air flow.)
Damping capacitors is very often beneficial. Caps sometimes suffer from microphonics as much as vacuum tubes do. Dampers can be used on appropriately sized and accessible caps. I've found Permatex Blue RTV Silicone Gasket Maker (available at auto parts stores) to be excellent for damping caps, sonically neutral and extremely heat resistant.
Mortite or other brand of rope caulk is most beneficial along the interior sides and bottom of the chassis, under the lid, around circuit board mounts.
Herbie's Audio Lab