Adding a unique mechanic only available trough just one item isn’t a good idea. Also it is just a prebuff like this.
Either do something like: while in either form, get bonuses according the other form skill level. Like when in werewolf form, get base level werebear bonuses, vica versa. The number needs to be tweaked to not be too OP, but be worth the runes. Maybe just add a percentage of the other skill bonus.
Or just add the mark bonuses as flat bonuses while in the form. Maybe even make the bonuses better. This is the lazier and more boring option, but it would serve a purpose.
Being able to teleport and transform directly between forms, etc, should be added regardless, among other QoL improvements.