i am thinking to solder my two lipos in parallel connection for a project

7.4 volt 1300mah 20-25c
and a 7.4 volt 850mah 15c battery

question is-what will happen to the c rating of the pack if connected in parallel connection?

please

you will fry the battery with the lower rating .......
explained as bellow
if you connect the two batteries than nothing will happen at that time , all will happen when you complete the circuit..
now the bat with 25c will give out the power of approx 250 watts
it means that a total of 32amps of current will be flowing , and the later bat give 90watts means a total of 12amps
when both the batt are connected in parellel than according to kirchoff's law of current the rest of current [32-12=20A]
will be flowing thorugh the low rating battery and this huge current will fry your battery......
its just a guess but results may vary , battery my blast , catch fire , or nothing will happen if my guess is wrong.....
plz reply me your results...

In parallel it should be OK, is my understanding, if they are same cell count and start at similar voltages. 

The C rating should be based on the lower of the two.

Gurtejtoor - Just because a pack is capable of delivering a particular level of current, does not mean that it does so automatically.  It depends on the load connected.  In a parallel situation, the discharging of both packs would be in such a way that the voltage on both of them would come down together equally, and they will get discharged at the same time.

Series connection is a bigger problem, if the packs are of different mAH rating :

http://www.rcindia.org/beginners-zone/using-cells-of-different-amh/

you right anwar , i always gets confused in parellel n series settings.. its really i big mess.
my answer would be right for series connection i guess.....