If I'm understanding the problem correctly, you have a fixed position page which contain 2 flowed suforms each with one expandable field. Thus, when the first field expands, it overlaps the second field since the first subform grows but it does not push the second subform down to make room.
If that's the case, changing your page's content type to flowed should fix your problem.
Also, you won't be able to get the field/subform height on an object that grows dynamically since it's height is always set to 0 (I ran into that problem earlier this month...).