smarsden:
We currently have homeline with Spark, with two Spark mobiles attached on their open term OneBill plan, at only $19 per month per mobile.
...
but then might also have to change to a different mobile plan with Spark. This will probably cost more since I think the price-point we're getting on mobile currently with Spark is only because our homeline is with them also.
You can just get the OneBill $19. You don't need to bundle it with anything else. (Although if you want to spend $19 a month with Spark, the prepay plan will give you better value.)