Make two piles. One has 90 pennies the other one has 10.
The 90 penny pile has X tails while the 10 penny pile has 10-X tails (and therefore X heads).
Turn over all the pennies in the 10 penny pile and now that pile has 10-X heads and X tails.
Now both piles have X tails. (Although X can be any integer between 0 and 10 inclusive.)