I played around with the max and I got the formula to work.

=SUMIFS('BW-Orders-Old'!$P$1:$P$25500,'BW-Orders-Old'!$A$1:$A$25500,">="&BN$3,'BW-Orders-Old'!$A$1:$A$25500,"<="&BN$4,'BW-Orders-Old'!$C$1:$C$25500,$A115,'BW-Orders-Old'!$D$1:$D$25500,$B115)+IF(BN$4>MAX('BW-Orders-Old'!$A$1:$A$25500),SUMIFS('BW-Orders-D'!$P$1:$P$10418,'BW-Orders-D'!$A$1:$A$10418,">="&BN$3,'BW-Orders-D'!$A$1:$A$10418,"<="&BN$4,'BW-Orders-D'!$C$1:$C$10418,$A115,'BW-Orders-D'!$D$1:$D$10418,$B115))