So according to the De brogli's we know the relation between momentum and wavelength which is 

$\mathrm{\lambda } = \frac{\mathrm{h}}{\mathrm{mv}}$

If both particle have same velocity but when wavelength of $\mathrm{A}$ is found to be double than that of $\mathrm{B}$

Then relation between mass of both particle is 

$$\begin{gathered} {\mathrm{\lambda }}_{\mathrm{A}} = \frac{\mathrm{h}}{{\mathrm{m}}_{\mathrm{A}}{\mathrm{v}}_{\mathrm{A}}} \\ {\mathrm{\lambda }}_{\mathrm{B}} = \frac{\mathrm{h}}{{\mathrm{m}}_{\mathrm{B}}{\mathrm{v}}_{\mathrm{B}}} \\ {\mathrm{\lambda }}_{\mathrm{A}} = 2{\mathrm{\lambda }}_{\mathrm{B}} \end{gathered}$$

Thus, relation is mass of $\mathrm{A}$ particle is half that of $\mathrm{B}$ particle