Power Query to import text file having two rows for each field

That's quite a challenge.

My suggestion would be to first create a table with the data of each first row and another table with the data of each second row.

Then combine both tables.

I created the following example file with part of your data (at estimated positions):

Assuming you need no data from the headers and assuming input is at fixed positions, I created the following code.
I renamed each step and otherwise I adjusted generated code (or created code using the advance editor) for the steps:
Source (adjusted in order to get only 1 column)

SplitByPosition1 and -2

RemovedTopRows1 and - 2 (remove based on condition)

CombinedAsRecords

let
    Source = Csv.Document(File.Contents("C:\Users\Marcel\Documents\Forum bijdragen\Power BI Community\Power Query to import text file having two rows for each field\Legacy.txt"),1,"#(lf)"),
// First the first rows
    SplitByPosition1 = Table.SplitColumn(Source,"Column1",Splitter.SplitTextByRanges({{0,13},{17,15},{35,15},{63,10}})),
    Trimmed1 = Table.TransformColumns(SplitByPosition1,{},Text.Trim),
    RemovedTopRows1 = Table.Skip(Trimmed1,each _[Column1.1]<>"A/C NO"),
    PromotedHeaders1 = Table.PromoteHeaders(RemovedTopRows1),
    Typed1 = Table.TransformColumnTypes(PromotedHeaders1,{{"A/C NO", type number}, {"Princ Bal", type number}, {"Int Accr", type number}}, "en-US"),
    RemovedErrors1 = Table.RemoveRowsWithErrors(Typed1),
    ResultPart1 = Table.SelectRows(RemovedErrors1, each ([#"A/C NO"] <> null)),
//  Now parse the second rows
    SplitByPosition2 = Table.SplitColumn(Source,"Column1",Splitter.SplitTextByRanges({{18,15},{50,10}})),
    Trimmed2 = Table.TransformColumns(SplitByPosition2,{},Text.Trim),
    RemovedTopRows2 = Table.Skip(Trimmed2,each _[Column1.1]<>"Interest Rat"),
    PromotedHeaders2 = Table.PromoteHeaders(RemovedTopRows2),
    Typed2 = Table.TransformColumnTypes(PromotedHeaders2,{{"Interest Rat", type number}, {"Fees Recv", type number}}, "en-US"),
    RemovedErrors2 = Table.RemoveRowsWithErrors(Typed2),
    ResultPart2 = Table.SelectRows(RemovedErrors2, each ([Interest Rat] <> null)),
// Now combine the results
    CombinedAsRecords = Table.FromColumns({Table.ToRecords(ResultPart1),Table.ToRecords(ResultPart2)}),
    Expanded1 = Table.ExpandRecordColumn(CombinedAsRecords, "Column1", {"A/C NO", "Borrower", "Princ Bal", "Int Accr"}, {"A/C NO", "Borrower", "Princ Bal", "Int Accr"}),
    Result = Table.ExpandRecordColumn(Expanded1, "Column2", {"Interest Rat", "Fees Recv"}, {"Interest Rat", "Fees Recv"})
in
    Result

This should help you to create the code for your file. If you need further help then please provide a representative sample file.

This is the result from my example file:

You may want to add some finishing steps, like reordering columns.

An alternative solution which is probably much faster is the following in which the source is duplicated in 2 table columns, with the 2nd column shifted 1 row.

let
    Source = Csv.Document(File.Contents("C:\Users\Marcel\Documents\Forum bijdragen\Power BI Community\Power Query to import text file having two rows for each field\Legacy.txt"),1,"#(lf)"),
// Add column shifted 1 row
    AddedShiftedColumn = Table.FromColumns({Source[Column1],List.Skip(Source[Column1],1)}),
    SplitByPosition1 = Table.SplitColumn(AddedShiftedColumn,"Column1",Splitter.SplitTextByRanges({{0,13},{17,15},{35,15},{63,10}})),
    SplitByPosition2 = Table.SplitColumn(SplitByPosition1,"Column2",Splitter.SplitTextByRanges({{18,15},{50,10}})),
    Trimmed = Table.TransformColumns(SplitByPosition2,{},Text.Trim),
    RemovedTopRows = Table.Skip(Trimmed,each _[Column1.1] <> "A/C NO"),
    PromotedHeaders = Table.PromoteHeaders(RemovedTopRows),
    Typed = Table.TransformColumnTypes(PromotedHeaders,{{"A/C NO", Int64.Type}, {"Princ Bal", type number}, {"Int Accr", type number}, {"Interest Rat", type number}, {"Fees Recv", type number}}, "en-US"),
    RemovedErrors = Table.RemoveRowsWithErrors(Typed),
    Filtered = Table.SelectRows(RemovedErrors, each ([#"A/C NO"] <> null))
in
    Filtered