Merging tables with blank fields

A solution in Power Query; maybe a similar approach can be done in DAX.

Create a temporary table (UniqueAValues) with the nonblank distinct A values from both tables.

Merge these with Table1 and next with Table2 and take the maximum B value from either table.

Query UniqueAValues:

let
    Source = Table.FromColumns({List.Distinct(List.Select(Table1[A]&Table2[A],each _ <> null))},type table[A = text]),
    #"Merged Queries" = Table.NestedJoin(Source,{"A"},Table1,{"A"},"Table1",JoinKind.LeftOuter),
    #"Expanded Table1" = Table.ExpandTableColumn(#"Merged Queries", "Table1", {"B"}, {"Table1.B"}),
    #"Merged Queries1" = Table.NestedJoin(#"Expanded Table1",{"A"},Table2,{"A"},"Table2",JoinKind.LeftOuter),
    #"Expanded Table2" = Table.ExpandTableColumn(#"Merged Queries1", "Table2", {"B"}, {"Table2.B"}),
    #"Inserted Maximum" = Table.AddColumn(#"Expanded Table2", "B", each List.Max({[Table1.B], [Table2.B]}), Int64.Type),
    #"Removed Columns" = Table.RemoveColumns(#"Inserted Maximum",{"Table1.B", "Table2.B"})
in
    #"Removed Columns"

Similarly for nonblank distinct B values.

let
    Source = Table.FromColumns({List.Distinct(List.Select(Table1[B]&Table2[B],each _ <> null))},type table[B = Int64.Type]),
    #"Merged Queries" = Table.NestedJoin(Source,{"B"},Table1,{"B"},"Table1",JoinKind.LeftOuter),
    #"Expanded Table1" = Table.ExpandTableColumn(#"Merged Queries", "Table1", {"A"}, {"Table1.A"}),
    #"Merged Queries1" = Table.NestedJoin(#"Expanded Table1",{"B"},Table2,{"B"},"Table2",JoinKind.LeftOuter),
    #"Expanded Table2" = Table.ExpandTableColumn(#"Merged Queries1", "Table2", {"A"}, {"Table2.A"}),
    #"Inserted Maximum" = Table.AddColumn(#"Expanded Table2", "A", each List.Max({[Table1.A], [Table2.A]}), type text),
    #"Removed Columns" = Table.RemoveColumns(#"Inserted Maximum",{"Table1.A", "Table2.A"})
in
    #"Removed Columns"

Append both temporary tables and remove duplicates.

let
    Source = Table.Combine({UniqueAValues, UniqueBValues}),
    #"Removed Duplicates" = Table.Distinct(Source)
in
    #"Removed Duplicates"

You won't get the sort order, but that shouldn't matter.