You need to take Utrogestan before you go to bed- that way, you won't notice any side effects. Dizziness and sleepiness are listed as side effects. I take mine very last thing after cleaning my teeth.
You shouldn't try to half them. If you read the leaflet it should describe what is in them- powder mixed with oil, so they will be very gooey if you cut them and you won't get an accurate 'half' because some may ooze out.
Some women use them vaginally - not licensed in UK for this but it's fine if you want to try as this is the 'norm' in France more than UK.
In any case- you can't really alter the dose like this unless you extend the number of days you take them.
If you do think that a smaller dose would help, then ask your dr because 100mgs exist, but you'd then need to work out how many days because you need more days at 100mgs than 200mgs.