so sorry things are so tough, particularly with two teen DDs struggling. I know how hard it was with one. Is there a link with both girls and their depression ? They seem very close in age. I know it was hard for my DS, when my DD was ill so my family advice would be to deal with them as individuals rather than siblings, and allow them to focus on getting themselves stronger rather than each other. (you are probably doing this) .
If DD2 is also struggling with school,I wonder has she had an ed psych assessment ?
I would hope that with two teens, Camhs would push you up the list a bit. Also if DD2 will not attend school, they do need to take notice. Update them regularly and hassle , if you are getting worried about DD2 and her low mood.
I dont know how to advise, other than I moved schools for my DD during recovery from severe depression which really helped, so it might be something to consider for the future, as it can make a difference to make a fresh start when they are feeling stronger a better.
Young minds, has a callback service, in case they can give you advice and useful website info . Relate seems to offer a childrens service in some areas now . I got myself CBT via the GP which helped me understand how to communicate in these situations. best of luck to all of you.