You have two errors in your code that exclude the possibility to have both trees on the same horizontal line:
- You have an empty line between the
forest environments which TeX interprets as a new paragraph. Remove that empty line or insert a
forest environments has empty lines in it. This prevents TeX from compiling your code at all as it breaks the
forest parser. The same rule as for above is valid.
You now have both
forest trees on the same horizontal line. As TeX sees the trees (the underlying TikZ pictures) as two big box which have their baseline at the bottom the roots of the trees are not vertically aligned.
You can change this with the
baseline option (a
forest option here, there also exist a TikZ
baseline option). At any node you use the
baseline option, the baseline of the whole tree is set. In this way, you can give the
baseline option to the
VP node in the first tree and the
Det node in the second tree to vertically align the pictures at these node. Is the option given as an option to the
forest environment, the option is applied to the root node.
If you want both trees in one picture (to make it easy to draw between them), the
phantom option might be helpful. You may need to include a few
phantom levels to align the sub-trees at different levels as in the example before.