I don't have much time to do my trainings, I just do my long run on Sundays mornings, but I would like training on Saturdays too, but maybe the training of Saturday could affect my long run of Sundays.. so I would like to know what could be a good 2 consecutive day training per week. I was thinking of doing a trail running session on Saturday, and Sundays a fartlek in my long run.
The only reason not to practice two days in a row is because the body gets fatigued, and a fatigued body will not have the energy and time to adapt to the training.
How does the body get fatigued?
When you perform one type of training, say long distance running, you are taxing one of several energy systems in your body, specifically, your muscles. In the case of long distance running you are taxing the aerobic system, which utilizes oxygen to make energy for muscle contraction. In the hours after long distance running, your body will undergo changes to prepare for the next session of long distance running with increased functioning. In this time period there will be more harm than good if you do the same training and tax the same system.
To answer your question, one way to circumvent this problem is to tax different energy systems on alternate days. That is, saturday you can do long distance running, while on sundays you can do an anaerobic glycolytic training such as strength training or sprinting.
This way your body will adapt more to the demands put on it, and you will lower the risk of becoming overtrained (Generally speaking. However, this risk is low when practicing 2 times/week).
On an additional note, the energy systems do differ in the time it takes for them to recover. This is exemplified by the phenomenon of supercompensation. Supercompensation is what occurs after the muscle has gotten adequate rest, and also adapted to the increased demand by increaseing fitness temporarily. It is when you practice again, in the time frame of supercompensation, that the body increases its fitness and you become stronger/more endurant. Aerobic training takes less time to regenerate (about 8h for low intencity running), while high-intensity anaerobic glycolytic traning takes longer and thus requires a longer rest period (up to 72h for competitive sprinters and weight lifters).