Hi,
That can happen if you have the two clips selected at the same time. If that's the case, make sure that when you adjust the second clip that the first clip is not also selected. When you put your cursor on the 25% clip it should be twice the time duration of the 50% clip. (The lower the time duration, the slower the clip.)
You can also adjust the speed with the speed editor handles that appear when you control click on the clip and select Show Speed Editor in the dropdown menu. So you can make a speed correction if necessary.
If still having problems try opening iMovie while holding down the Option and Command keys and selecting to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. Now see if the speed adjustment works properly.
You can also try putting your clips into a newly created project to see if that cures it. Sometime glitches occur that are confined only to one project.
-- Rich