The “subscription” being your Silver license, in this case. We refer to paid plans on GitLab.com as such. If you have two separate groups on GitLab.com with the same users in both then you’ll be paying for each user twice, once for each group.
If the primary function of the second group is to only serve as a backup pipeline minute repository, then it sounds like you may end up overpaying and it would also be difficult to run your pipelines on the other group if the minutes for the first are exhausted.
It seems like the upcoming add-on minutes feature would be really beneficial for you if you need more than 10,000 minutes per month and Gold is not an option.