Randy,
Are you talking about two people being able to change, for example, the layers in a web map? Subscriptions will not provide this ability. We have discussed the notion of shared ownership of items, which would allow multiple people to edit it, however there is no current plan to implement this feature. However, we are aware of the need, especially with respect to group, where everyone might want to edit an item.
If you are talking about editing features, then shared editing of features is possible if the layer in the web map references a feature service.
Thanks,
Mike