Is it possible to visit Xiaolangdi and white horse temple on the same day, from Luoyang city ?

One day is totally enough to cover the two spots.
As I know, you can take a bus to Xiaolangdi opposite Luoyang Railway Station and you will arrive there after 1h. The recommended visiting time is 1~2 hours. After that, still take that bus to go back to the city proper. Upon arrival, you can either take bus no. 56 in front of the bus station to the terminus, White Horse Temple, after 1 hour's ride, or get on a taxi to go directly to the temple in around 35min at a cost of CNY35.
May help!