If for example BLOCK_NAME is your multi text item, TEXT_ITEM1 is your first text item and TEXT_ITEM2 is your second text item, you can write the following code in when-button_pressed
:
go_block('BLOCK_NAME');
first_record;
:BLOCK_NAME.COLUMN1 := :TEXT_ITEM1 + :BLOCK_NAME.COLUMN1;
last_record;
:BLOCK_NAME.COLUMN1 := :TEXT_ITEM2 + :BLOCK_NAME.COLUMN1;
commit;