When they can roll over themselves, then it's ok to let them choose their own preferred sleeping position. Just be sure to use light-weight coverings, and when you put them to bed don't pull the covers up too far.
If you live in a cold climate, put more clothes on the babies, rather than heavier coverings.
At 7 months, it's safest if the babies can comfortably and safely move around on their own. It would be unsafe to try to restrict their movement to keep them on their backs, as it may simply cause them to get more tangled in the bedding while trying to move.
Just like with adults, older babies need to be able to move around in their sleep, to limit pressure on any particular part of their body. This isn't such an issue for small babies, as they're typically only sleeping for a few hours at a time. However for older children, when ideally you'll want them to be sleeping through the night, it is very important that they are able to move around.