It cannot be done as you have described, but if you are willing to change your requirements somewhat, I think you can have a workable solution.
All calendars are public or private. If you make this stake calendar public, then all the wards can see it. You can name it such that other wards won't be tempted to subscribe to it (e.g., "RV/10th YM/YW Activities"), and assign editors among the youth leaders from both wards as appropriate. You'd also probably want to add at least one of the ward administrators from each ward as an editor. This seems like the best solution, even though it doesn't meet your requirement to have it visible only to the two wards.
The other option is to make that stake calendar private. That will enable you to add as viewers only the members of the two wards. But that seems like a hassle initially (adding as viewers at least all the youth, their parents, and their leaders) and for ongoing maintenance as people move in and out of those two wards or change callings, or move into or out of the youth classes because of age. Specifying the editors would be the same as described above.
Questions that can benefit the larger community should be asked in a public forum, not a private message.