The Collaborative International Dictionary of English v.0.48:

Interscribe \In`ter*scribe"\, v. t. [imp. & p. p.
   Interscribed; p. pr. & vb. n. Interscribing.] [L.
   interscribere; inter between + scribere to write.]
   To write between. [R.]
   [1913 Webster]