pdf manupulation problem

Question

i am using spire.pdf

i have a multiple A4 size pdf file

 

i want to each pdf file in 2 page up in A3 size page

 

but little condition

 

such as pdf file is 99 pages

 

1 page no. to 50 page no. is left side of A3 Size page

and 51 page no. to 99 page no. is right side of A3 Size page

 

i an unable to do it

 

please help me