It is enough if Tiruvaradhanam is performed once a day for Salagrama and other Perumal deities. It is special to perform Tiruvaradhanam second time during the nighttime. One must be pure to perform this Tiruvaradhanam. Even when the second Tiruvaradhanam is not performed, we must at least offer milk during nighttime. We can offer the food items that we partake, if not at least milk must be offered.
There is no harm if we are unable to perform Tiruvaradhanam second time for Salagrama. Also, there is no compulsion to do so.