If it were 2 Helix units or a Helix and xx9 series unit, it there would be no problem doing this.....and i am pretty certain you could share the GPS from the 898 to the Helix ..... but there may be problems going the other way.
I emailed Humminbird about transducer sharing a year or so ago, and they told me "the new can see the old, but the old cannot see the new." It may be that the GPS would share, but would be quesionable.......Someone who has that setup is going to need to test this to see if it works.