Alek, These two woods, being whitewood, should be cut during the growing season.IMO Only at that time will the bark slip and all you do is peel the bark off and whats under is the pristine back for the bow. Any wood can be cut at any time of the year but during this time of the year the bark holds fast and it takes a lot of work to get it off and in many cases you will have to chase the next ring to get a good solid back ring unless you are very careful. On woods like osage, locust and mulberry, where you generally remove the bark and sapwood anyway, winter is a good time to cut because of less bug, snake and heat problems.
I prefer to harvest whitewoods as early as possible in the spring(after the leaves have come out)so I have last years annual ring for a back. It has had all winter to mature. If cut in late summer the present years growth ring has been laid down but, in my opinion, has not had time to mature.
If you wait until spring to harvest these woods you can take a few staves down to floor tiller stage and they can be ready to work in a month or so. Pat