Ratio of spring constants of the two spring segments = 2:1 (larger segment will have smaller spring constant)
So consider the values to be 2x and x. When these two are in series, they give rise to the original spring. So
2x * x / (2x + x) = k
which gives x = 3k/2.
So spring constants are 3k/2 and 3k.
When they are in parallel, effective spring constant = 3k + 3k/2 = 9k/2
Time period T = 2*pi sqrt (2m/9k)