Marc,
You should wait a while yet. I usually do not see sufficient roots to pot up cuttings until they have 3 or 4 sets of new leaves. Early spring hardwood wood cuttings for Japanese Maples (almost only Arakawa) that I take are usually ready for potting up in the May- early June time frame. I think your climate might be a little earlier than mine, and with your generous rain and humidity you should be fine separating them at that time of year. Good luck!
Regards,
Martin