Thanks for the query.
It is difficult to tell exactly who is the father of the child.
According to the history, possibly the first person could be the father as you are thinking.
But sometimes pre ejaculate fluid can contain few spermatozoa
In that case there is possibility for the second person also.
If both the acts occurred same time, it is difficult to guess.
If there was difference of few days between the two acts, by calculating the ovulation dates, one possibility can be drawn.
Usually in case of regular 30 days periods, ovulation occurs on around 14th to 18th day of the cycle.
Best option is going for DNA test.