How can I attach an in-memory SQLite database in Python?

I'd like to merge SQLite databases, and some may be in memory. I create the in-memory databases by specifying the database path as :memory:. Following this post, using the attach feature of SQLite seems like both a simple and efficient approach. But how can I specify my in-memory database as the source to attach?

For example, I'd want to do something like:

c1 = sqlite3.connect(":memory:")
c1.execute(...create table, insert a bunch, commit...)

c2 = sqlite3.connect(":memory:")
c2.execute("""
  ATTACH ? AS ToMerge;
  BEGIN; 
    INSERT INTO Records SELECT * FROM ToMerge.Records; 
  COMMIT;
""", (c1.get_attach_id(), ))

but, of course, c1.get_attach_id() is a method I made up for demonstration purposes, since using the string :memory: would be ambiguous. How can I specify the existing c1 database?