  Using a numerical decimation method, we compute the localisation length
$\lambda_{2}$ for two onsite interacting particles (TIP) in a one-dimensional
random potential. We show that an interaction $U>0$ does lead to $\lambda_2(U)
> \lambda_2(0)$ for not too large $U$ and test the validity of various proposed
fit functions for $\lambda_2(U)$. Finite-size scaling allows us to obtain
infinite sample size estimates $\xi_{2}(U)$ and we find that $ \xi_{2}(U) \sim
\xi_2(0)^{\alpha(U)} $ with $\alpha(U)$ varying between $\alpha(0)\approx 1$
and $\alpha(1) \approx 1.5$. We observe that all $\xi_2(U)$ data can be made to
coalesce onto a single scaling curve. We also present results for the problem
of TIP in two different random potentials corresponding to interacting
electron-hole pairs.
