Why do Tarjan's and Kosaraju's algorithms for finding strongly connected components have the same running-time complexity?
they say that both have O(|V|+|E|) time complexity.

that didn't make sens for me since Kosaraju use two dfs pass and compute transpose graph, but Tarjan's use only one dfs

If you check the definition of Landau notation you will see that it does not say which function is smaller, that is (here) which algorithm is faster. It ignores constant factors.
Both algorithms have running times in $\Theta(|V| + |E|)$; that does not preclude one of them being $2^{100}$ times faster than the other for all $n \leq 2^{100}$.