Can a planning tool work while we are running two ERPs in parallel?

Mixed landscapes are the norm rather than the exception, particularly in groups with many legal entities where a full migration would take years. The usual design is to consolidate the sources upstream, either through a data warehouse or through parallel feeds mapped to the same planning model, so the planning layer sees one coherent picture regardless of which system each site sits on. The practical constraint is data consistency rather than the number of systems: units, product identifiers and calendars need to reconcile. That mapping work is part of a normal onboarding and integration phase and is worth confirming against your specific landscape early.