Controling Voltage percentage wise
It's not clear what exactly you are taking 10% of to determine the maximum allowed difference between the two signal.  I'll assume that you want to know whether signal B is within 10% of signal A.

There are several ways to do this.

The caveman way

Use a diff amp to make the voltage difference between the two signal.  Use a voltage divider on signal A to make the max allowed voltage difference.  Just comparing the two isn't good enough since the diff amp output is &plusmn; and you only have a single-ended maximum allowed threshold.

One way to solve this is to follow the diff amp with a absolute value circuit.
Then you compare abs(B-A) to the threshold with a comparator.

Another way is to make the negative of the maximum difference signal.  Then use a window comparator to detect whether (B-A) is within the &plusmn;threshold range.

The 1990s way

Use a microcontroller with two A/D inputs.  Measure the two signals and do the calculations to get the answer.  This is the way to go as long as the signal bandwidth and response time requirements are low enough.  10 &micro;s would be easy to do with a number of mainstream micros available today, for example.