Speaking of biscriptalism, Guy Almog called my attention to an interesting project called Aravrit (that is, Arabic + Hebrew [ivrit]).
From the home page:
Aravrit is a project of utopian nature. It presents a set of hybrid letters merging Hebrew and Arabic.
This new writing system is composed of an Arabic letter on the upper half and a Hebrew letter on the bottom half. The characteristic features of each letter were retained, however in both languages the fusion required some compromises to be made, yet maintaining readability and with limited detriment to the original script. In Aravrit, one can read the language he/she chooses, without ignoring the other one, which is always present.
Judging from Aravrit's Facebook page, many of the details of this new, hybrid script were inspired by features found on Yemenite manuscripts.