As far as I aware, there is no impact until / unless if you have 2 way trust relationship between forest domains to sync those machines to same ePO through different registered servers to load the systems to deploy the agent from ePO.
If you have other way of handling the MA install like through SCCM, GPO, login scripts or so.
There is no impact, since either way if the machine installed MA, it works through the managed agent concept & removes the dependency of the machine is member of domain or work group.
Other than this, If you have policy assignment rules with user based or system based policy, which it should be reachable to access the domain of another forest from ePO console, hence 2 way trust relation is a need.
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