Replace missing observations in VAR1 with observations from VAR2 f

Dear forum,

I am trying to replace the missing observation(s) in ExportMarkets with the value in MirrorExportMarkets. May I ask what the command is for this?

ExportMarkets MirrorExportMarkets
1991 . 45 67
1992 . . 58
1993 . 97 89

Kind regards,
Ray