multi part SMS using at command

hi guys
a confusing situation , i just wanted to send a multi part SMS .
as i know i can't do it in text mode
i'm able to send a one part SMS in PDU mode , now
but i don't know what should i do for the multi part
as i know , i have to add the UDH in the begging of the text of my PDU .
i have done that , and i have set the bit for the UDH in the first octet of PDU .
but now i don't know if i have to send two different SMS in pdu mode , and the receiver will join them together or what ?
the main question is that what is the proper number to send for AT+CMGS , should it be the size of both to message or each of the part of the SMS should be apart or what ?
i'm using the 16 bit mode for characters , so i just can send 140 octet , which mean 70 character total .
when i add UDH it will need 6 octet for itself , so should i just send the same number for any first SMS in any other SMSes too ?
so0 , any help ?
i don't know if there is any thing that u should know :-?
i'm using sin900 gsm module and thats all i think .
thanks a lot