If you are not an elite marathoner it is really not necessary to train twice a day. I run 95km a week on single runs using the following schedule :
Monday 10km easy (4:25min/km) Tuesday 5 x 2000m at 10km pace (3:25min/km) Wednesday 15km easy (4:25min/km) Thursday 5 x 1000m at 5km pace (3:20min/km) Friday Rest Saturday 32km or race Sunday 15km easy (4:25min/km)
Total distance = 95km
This schedule has produced 34:20 10k and 2:42 marathon. See, no need for twice a day training. I train early in the mornings before work because I find it impossible to perform proper speedwork after a stressfull day.