IF U HAVE 45 ML SAMPLE OF CLEAN URINE (NEGATIVE ) AND U ADDED 5 ML
MORE OR LESS OF DIRTY (POSITIVE) URINE CONTAINING METHAMPHETAMIN WILL
YOU TEST POSITIVE.WITH A SPLIT SAMPLE WILL BOTH SAMPLE READ POSITIVE

I would think that this can't be answered without knowing how much
methamphetamine is in the "dirty" urine and precisely what tests are
being used.

it depends upon the lower limit concentration test limits.  most drug
tests set a lower limit saying that the [drug]<5mcg/dL for example. 
this concentration would depend on the amound of meth contained in the
dirty urine.  If there was very little to begin with and the
concentration became very dispersed it would be so minimal that most
drug tests would consider this a negative result.  Conversely if there
was a high concentration of meth, even upon dilution there is still
enough to cause the overal concentration to be higher than the minimum
allowed concentration yeilding a positive result.

The lower limit for methamphetamine for the department of
transportation drug tests is the lowest accurate measure of 200 ng/mL.
(http://jobsearchtech.about.com/library/weekly/aa090301-6.htm)   The
government is attempting to lower the concentration in urine to be
approximately 100 ng/mL, but accurate tests have yet to be developed
and implemented.  If a 45 mL sample were mixed with a 5 mL same of
'dirty urine' the concentration in the bad sample would have to be
less than 2000 ng/mL.  It is difficult to determine how much
amphetamine would have to be consumed to reach this urine
concentration, but i would suspect it would be a rather small amount. 
If you assume a person of 175 lbs having approximately 75% water
available for dilution of the drug, that would be about 131.25 lbs of
water.  2.2 lbs/kg= 59.66 kg 59.66*1000=59660 g of water.  density of
water = 1g/mL= 59660 ml of water available for dilution in the body. 
If the urine to body water concentrations were equal which they are
not because urine is concentrated body waste a person could have about
.119 g of amphetamines in their system.  But since urine is more
concentrated source of wastes say 5x, a person could have about .0239
g of amphetamines in their system at the time of urine expulsion;
however, it would be very difficult to determine how much amphetamine
and how concentrated the original mixture was.  This is all an order
of magnitude estimate, but this should help get a clearer picture.