It does sound like your older child could move into Bigger Hearts or Preparing Hearts. Without knowing more about your 9 year old, I'm not sure which program would fit him better with all combining scenarios set aside for now.

If you haven't already checked the placement chart for your 7 year old, you will want to do that to be sure that your 7 year old could also handle Bigger Hearts. If your 7 year old doesn't truly fit in Bigger Hearts, you could instead consider combining him with the 6 year old in either LHFGH or Beyond,depending on where the 6 year old fits best.

If your 7 year old does fit well in Bigger Hearts, then combining the 7 and 9 year old in Bigger is a good choice.
Here's a link to placement chart.
http://www.heartofdakota.com/placing-your-child.php
Skill level makes such a difference in correct placement, so it is a good idea to place each child on his/her own first and then see who could possibly be combined and in which program.

If you're willing to come back and share a bit more about each child, we could advise you even better and talk through possible options with you.
On a sidenote, we actually plan for Bigger Hearts to come after Beyond as the two complement each other so well in their study of American history. Beyond is about cultures and people groups who settled in America and their reasons for coming. It also gets into early colonial events and the founding of the nation as well. Bigger Hearts is a biographical study, with a focus on the people. The events are happening in the background of the lives of the people about whom you read. So, the two guides have a totally different feel.

We've never had anyone say the two guides were redundant, although parents often worry they may be.
Blessings,
Carrie