Check and see if your cell provider has the ability to offer combined mailbox service as an option.
My cell and landline are from the same ILEC telco provider (BellMTS) and they have the ability to have calls from either number go to one mail box and both notify my cell and light the call waiting display on the landline sets. The cost is $5 monthly on the landline unless you have a service bundle you can put it in.
I have never asked but I don't see why it could not be done for two cells or two landlines to have combined voice maibox service.
Clifford Lewis