It's not truly supported, but you could do something like it in theory.

1) Pass save file back and forth using e-mail/messenger or folder sharing.
2) One player keeps password protected room up, joining players can manipulate pieces and the changes will stay as long as the room is up.

I haven't seen it suggested yet. You might want to post a suggestion on the Tabletop Sim forums (They're no longer taking suggestions from Steam forums).

also maybe you could get it working by creating the save file as a mod in workshop and adding the other player as co-creator and every turn just "update" the "mod" one after another