How do you create multiple masks?

I’ve been getting very frustrated with flash the last hour or so hehe, not sure whether its just me or not but I was trying to apply two masks to one layer and i just cant seem to do it.

I created two seperate motion tweens ( a layer on there own ) which involved animated masks which would hopefully reveal my main page which is on the layer underneath. Example

>Motion tween layer ( animated mask coming in from left to the middle )
>Motion tween layer ( animated mask coming in from right to the middle )
>Background image

Now I would have thought it was easy enough to link or merge the two tween layers together and then apply a mask but flash doesn’t allow any form of merging or even grouping to which i could apply the mask. So… i went through loads of hassle of trying to combine the two tween layers in a movie and then put it above the background as a single layer which proved to be a horrific nightmare!

Is there some easy option that I am missing or are masks really this frustrating to use.

Thanks a lot in advance!

Tris