Split number in two, so that two rounded portions equal the original. Say I have number $x$ which can be split into $y$ and $z$ to with the split being a percentage of $x$ to unlimited precision.
How do I ensure the sum of $y$ and $z$ will always sum back to $x$ after rounding?
For example:
$x = 10.1$ with split = $50\%$
$y = 5.05$
$z = 5.05$
rounding to 1dp
$y = 5.1$,
$z = 5.1$
$y + z = 10.2\neq x$
 A: You don't.
When rounding you change the value to an approximate one so when summing back you cannot always go back to the original value.
However you can artificially do it. Whenever you round a number, you either lose or gain a certain value so I suggest doing it slightly differently:

Split $10.1$ into $y = 5.05$ and $z = 5.05$. When rounding both value automatically gain $0.05$ making the sum $2\times0.05 = 0.1$ too large.
So when rounding $y$ to $5.1$, notice that you are $0.05$ too large and you will need to balance on another number to make sure the sum doesn't change. So then you need to "round" $z$ to $5.0$ making the sum $10.1$ again.

A: Some fascinating questions of consistency and purpose emerge:
If we're content with an approximate split, why should we want it to recover to exactly the original number?
If we're content with 5.1 as an approximation to 5.05, then why not 10.2 to 10.1?
If the purpose is to reduce bothersome decimal places, then why not split 10 into 5 and 5? 
(Hopefully the last answers the original question).
