If we sync the same rows to two different destinations, will we be charged twice, or will the MAR count stay the same?
Each connector will incur MAR independently, regardless of whether the connectors are pulling from the same source. MAR is based on the number of active rows that are synced to the warehouse, not the number of rows pulled from a single source. Therefore, each connector will create its own MAR based on the modified rows written to the warehouse.
However, Fivetran do offer a free trial period of 14 days for new connectors in your warehouse. During this time period, credits will not be consumed.